Cheers is a fairly common valediction on e-mails sent by academicians and is often picked up by grad students almost instantly, to say nothing of Brits, Aussies and Canadians. I guess there’s not much overlap between the U. Colorado’s faculty and graduate students (or expat communities in Colorado from other parts of the Anglosphere) and Rotkovich’s clientele or he’d not have thought it strange, much less “very, very strange”
No wonder a normal American wasn't impressed.