I'm pretty sure the Cingetos aren't Druids per ce, but a sort of elite "temple guard" with close associations with them.

Anyway, "palace guard" units like the Praetorians, stationed in an imperial capital far from the constant low-intensity action of the ever-troubled borders, have always tended towards a nasty tendency to becoming "parade units" of rather lower military calibre than their running costs and prestige would make one except, with entry requirements having a bad habit of degenerating into issues of pure politics, favouritism and brown-nosing (not to say outright bribery...). Although I understand the Praetorians were relatively well able to maintain their military usefulness.

That being said, Rome in the 3rd century BC was at war with Carthage, Illyria,Macedon, Etruscans, etc. etc. Who were the southern Gauls at war with? Yes they raided one another but to my knowledge there was no major infighting during this time. Did the Gauls have a low birth rate or something?
If you mean the Cisalpines, come on now. They were like the Po river valley vs. the whole rest of the peninsula, with no quarantees of any kind of assistance from either the mountain tribes or the Transalpines (who in any case were busy fighting each other and the Germans). If nothing else the Romans were able to wear them down by sheer attrition over time, nevermind now "turning" them bit by bit to their own side. It doesn't take great genius to realize being a subject-ally tends to be preferable to being eradicated, all the more so given the rather Assyrian approach the Romans had to intractability. Not that a very vae victis approach to defeated foes wasn't rather more the norm than the exception those days anyway, but I understand the Romans were fairly good at exploiting it as a psychological weapon.

As for the Mediterranean coast of southern Gaul, the fact the Romans could easily ship in armies from Italy and the close trading links might have something to do with the way it fell into Roman orbit quite early on. Plus weren't there a few major Greek trading colonies like Massilia there, independent of the major Gallic powers and by what I've seen mentioned of them in passing relatively willing to enter into associations with major powers (both the Romans and Carthaginians had such client-cities in Iberia as well, I understand) ?

The numbers do count against the devastation claim of the civil war as it shows that there were plenty of people around and were not completely ground down due to civil war. In history there are many peoples who received their veteran status through war. If the Celts were battling amongst themselves they would have vital war experience. If these wars are long drawn out events then training would take place in addition to the war experience. During these times it is rare for entire armies to be wiped out. If these people had enough people to continue these wars they would be getting war exp.
You're missing the point of the division of labour in Celtic society and warfare. The fighting was done by the warrior class (and mercenaries); the commoner levy only became involved in dire emergencies (primarily the defense of their homes, I understand) and was not normally mobilized for offensive operations (with the possible exception of the segments providing the missile troops), ie. not only was their training rather poor but whatever combat experience they might earn was very sporadic, often quite brief and dismal, and quite possibly finished with getting massacred or sold into slavery.

Do recall that the Celts did not ascribe to the same sort of fully tribal warfare as the Germans for example did.

If you look into the Mogadishu (blackhawk down) situation, the constant warfare and experience is shown. The U.S. expected little resistance from the Somalia's because they didnt really have a standing army. What they forgot to take into account is the constant battling between the warlords which gave the Somalia's battle hardened veterans.
Oh come on. The warlords had thousands of irregular Tupac Army gunmen who had to chew drugs to deal with their fear, and had never been told blasting away at full auto is chiefly a good way to waste ammunition. The fighting lasted for something like half a day, and the total American casualties amounted to under twenty.

You try to do battle with comparable forces against an opponent of comparably vastly higher calibre in the context of premodern warfare, and what you most get is an utter massacre. Which in fact seems to more often than not have been exactly the result whenever the Celts had to try to fight the Romans head on mainly with their tribal levies, and duly why for example (AFAIK) Vercingetorix's strategy was one of harassement and guerilla warfare, not pitched battle (which in turn Caesar did his best to force). Any rag-tag bunch of bandits and angry natives can pull off frustrating guerilla warfare well enough.

I know the Arverni Guard were tough but I dont ever remember reading about Caesar avoiding a head on battle as he had tried to get Vercingetorix into open warfare.
I'd say JB was talking tactically - that is, contain and thus render impotent such elite heavies while the rest of their army is destroyed, after which it's entirely irrelevant how badass they are. Strategically Caesar of course wanted to force a decisive pitched battle where his superior troops could tear the heart out of the resistance (and scare the rest into submission), rather than try to manage an intractable province crawling with elusive and persistent bandits and guerillas.