If most sinus infections are viral vs. bacterial and can clear on their own- then how did my sinuses cause bacteremia for me? must've been bacterial?

Correct: If an infected sinus indeed was the source of your bactermia, then then you had bacterial sinusitits. Your own question acknowledges that "most" sinus infections are viral, or at least start that way. But bacterial sinusitis isn't rare, either initially or as a secondary infection that started as viral; or as a complication of allergic rhinitis with sinus blockage.