> "That's what I understand if I see "Thanks" instead of "Many thanks!""

That probably says more about you, or the particular dialect of your region, than him. "Many thanks" is not the predominate phrase for expressing thanks anywhere in that I have been in the continental US.

"Thanks" is only passive aggressive when in a context that makes that clear. It is most often used sincerely. That word alone is never "the tell" for insincerity; something else about the situation is.

You see "Thanks" as passive-aggressive? To me it reads as a very straightforward statement that communicates a moderate level of real gratitude.

This was a joke guys.