Thanks for your post. You are the first person I've seen from the "he pulled the punch" side of the fence that I think has across with a rational argument.

He may have lowered the Pit Fiends HP mid battle, especially as you pointed out that it had served its purpose.

However, do we know if Ghurrix is exceptional among Pit Fiends? Not every bad guy Vox Machina faces is going to be a hulking specimen or literally the strongest of his kind. Matt could have buffed the Pit Fiend by 50% (taking him to ~450HP) and said "that's good enough". By comparison, the Chroma Dragons are supposed to be well beyond the norm of a dragon. They were probably the 4 strongest dragons living, so of course they are powerful. The Pit Fiend may just be a Pit Fiend, one of many.

Plus, not all difficulty has to come from HP and damage. A lot of the difficulty in this fight was locked up in the hard time limit. Vox Machina had maybe another turn or so before a bunch of Efreeti join the party which would have crushed them easy. Not only did they have to win, they had to win quickly.