That's possible, but there are other possibilities/factors.
When I got my bearded dragon, most of what I read said that males grow to a length of 20"-24". I got a fecal shortly after getting him, which was negative, and while there was a period of "weed growth", he stopped growing at around 18". I can't say there wasn't just a
little disappointment, but that's long-gone.
Upon further investigation, it turns out (I'm told by experienced keepers on the BD forum) that average sizes have decreased over the years with generation upon generation of captive breeding.
There's also (albeit remote) possibility of dwarfism.
I'm not suggesting that either of these things is going on with Poseidon; I'm just saying there are other possibilities,
including the one you're asking/concerned about. Maybe even a combination, like genetically he might have turned out a little smaller than average—combined with some stunting from the parasitic infestation.
Bottom line, what can you do about it? I don't think it's your fault or anything—sounds like you did all the right things. Poseidon is what he is, and if he's otherwise healthy & (as far as anyone can tell) happy... enjoy him.