> The argument is correct, or it isn't, no matter who said it.
Yes, but we all have insufficient intelligence and knowledge to fully evaluate all arguments in a reasonable timeframe.
Argument from authority is, indeed, a logical fallacy.
But that is not what is happening here. There is a huge difference between someone saying "Trust me, I'm an expert" and a third party saying "Oh, by the way, that guy has a metric shitton of relevant experience."
The former is used in lieu of a valid argument. The latter is used as a sanity check on all the things that you don't have time to verify yourself.
I think its kind of like technical indicators. Obviously they mean nothing but because other people believe them you have to take them into account. So when someone with authority says something assertively many critical thinking faculties go out the window for many people