Well, to be honest you shouldn't take it that literal. I wrote it like that to make a point.
But I doubt the Roman watching the hoplomachus devastate other gladiators felt he was particularly overpowered. He was a man, and as a man he was attributed the victory.

"Ahhh... Yes Decimus pwns every time, he is simply teh roxxors!!!!11111!!!" Rather than any comment on the hoplomachus as a kit.

However over time the Romans refined the gladiator system, made it more streamlined, longer fights (armour not protecting your life, merely protracting the fight) ect ect. Obviously a man with a small shield and longer spear would be more fun to watch than a man who looked much like a hoplite.
It would be more mobile and a lot more gory (lots of small wounds ect, rather than single thrust and that was it).

So it was never a contious effort to balance the gladiators as the strength were believed to lie with the fighters themselves. But in the end the results were the same. The spearmen had been nerfed.