Subject-matter experts, such as distinguished professors, are considered reliable sources regardless of where their statements or writings are published. I'm not sure if it has always been this way, but it is currently written into Wikipedia's policy:
https://en.wikipedia.org/wiki/Wikipedia:Verifiability#Self-p...
You're right about fan sites. It's harder for people outside of the fan community to evaluate whether a fan site is reliable, and that contributes to the skepticism you see.