Let's do a little review here.

Number of purely greek factions in the game
Greek cities
Macedon
Seleucids
total 3

Number of might as well be greek factions
Thrace
Pontus
Egypt
total 3

Number of factions that are not greek but have a phalanx unit
Armenia
Germans
Carthage
total 3

Viewed like this the only factions that it might be pushing it for them to have a unit with phalanx ability are the bottom three on my list. Also you should not that CA only put in one blanket form of phalanx. There were at least 2 greek styles. The traditional southern greek phalanx and the Macedonian phalanx that Philip developed for his sarisafors to use with their long spears and small shields. Plus you have the shiled wall which is probably what the Helvetii were using anyway.