I interpret this to be a statement of the type "You should believe SIAI's claims (1) and (2) because we're really smart."

No, it's a statement of the type "You should believe SIAI's claims (1) and (2) because we're really rational." Your mathematician may have been smart and not rational. I remember reading about the phenomenon of smart non-rational people, maybe here: http://www.magazine.utoronto.ca/feature/why-people-are-irrational-kurt-kleiner/

Anyway, your mathematician is a terrible example of an irrational person because he was acting more rationally than any of his colleagues. Ethan Herdrick: "Our current known reserves of unapplied math should last centuries." Your mathematician's only mistake was not looking for a third alternative--a use of his time better than both math and environmentalist protesting.