Why the heck did they name them 'Soldier ants'...

Why the heck did they name them 'Soldier ants' and not 'Combatants'?

This is a one-liner joke that is also a pun. Soldier ants are ants that are specifically differentiated in order to make them better at combat. Thus, "Combat ants" would be a reasonable name for them. "Combat ants" sounds a lot like the word combatants, which means participants in a fight.

This is a fairly funny observation in my opinion.