Claiming it's an argument from authority is the fallacy fallacy.
An argument from authority goes: "I am an authority therefore the following is correct." This is nowhere near that.
He correctly stated it was his professional opinion that..... He didn't claim the argument is correct because he's a professional. And he is a professional, doing this work in a professional context for 15 months.
So it is absolutely correct to state "my professional opinion is...". He has likely put far more work into this, and certainly with better background and training, than probably anyone on this thread questioning his credentials instead of addressing his message.
An argument from authority goes: "I am an authority therefore the following is correct." This is nowhere near that.
He correctly stated it was his professional opinion that..... He didn't claim the argument is correct because he's a professional. And he is a professional, doing this work in a professional context for 15 months.
So it is absolutely correct to state "my professional opinion is...". He has likely put far more work into this, and certainly with better background and training, than probably anyone on this thread questioning his credentials instead of addressing his message.