Statcounter works by collecting and processing user agent strings from browsers accessing sites that use their analytics. Unknows OSes are probably the ones who don't give that info.
A lot of people who work in those call centres are uneducated and don't know shit about tech, they're only parroting off a script. In those call centres they use windows 7 still and don't even know about the existence of linux. Also piracy is big in India so they don't care if linux is free. And most of all their victims/"customers"(old people) don't use linux so they have no reason to either. It's just the fellow Indian nerds using Linux.
I saw one where a guy used linux mint and the scammer used the usual tree command but other than that he knew fuck all what he was doing. He was tryna look for notepad. It was hilarious