(Amusingly, my Google job offer landed in my Gmail spam folder years ago. But that was because of a weird thing where forwarding from my @university.edu account would screw with headers that Gmail uses to verify the authenticity of an email.)
Also...
> They must be using Gmail client to access their office emails, I guessed.
Is that even a thing? Does Gmail reroute stuff to spam that came into your inbox via POP/IMAP?