Hoechlin’s opinion of how he plays his character has no more weight than the interpretation of the text does.
The fact that he plays the character as having PTSD despite not intending to do it is likely to because of the text itself! Jeff Davis could and probably has written Derek as having PTSD, just because he’s never pointed it out to Hoechlin doesn’t mean it’s not intentional, likely Jeff trusts Hoechlin to play the character right as having PTSD without needing the complication of imposing on himself the constraint it would be to play a victim of PTSD.
Sometimes an actor knowing too much can be a hindrance to the text the author wants to convey, and Jeff Davis - Criminal minds anyone? He even openly admits to consulting with his brother who is a professional in the psychiatric field.
It’s a meta-interpretation, interpreted based on the behaviours of the character we’re shown in the text of the show.
Saying Hoechlin doesn’t trust JD to do a decent job of writing it, THAT is a fandom assumption.
Same goes for Dylan O’brien, he may not know or intend to play his character bisexual, it doesn’t matter if he turns out to be bisexual. The actors admittedly know nothing of what is coming, they only get their parts of the next script they are doing.
Bisexual Stiles is interpreted like Derek’s PTSD through what we’re shown in the text, there are too many examples that supports a reading of these things.
Furthermore, The sexuality of all characters except Scott is up in the air, they are never defined, reading his character as straight is a heteronormative bias, the same goes for the rest of the characters whose sexuality is not defined.