I was skeptical too ( I just got my series 7 from the US) . But here’s what I know:
-> Apple Watch has two models in cellular one in US and Canada and the other for the rest of the world.
-> The two models use different band chips for the cellular version and so the general rule is that it’s enough if the sim carrier company supports only that specific band pertaining to their country .
-> This is what happened to Airtel as well, their sim supported only the Indian version (Indian and the rest of the world). But after series 5, their sim supported all the models of the watch (both US one and the rest of the world).
-> Since your watch is SE which is newer than series 5, Airtel sim supports it.
And many people aren’t aware that Airtel has support for series 5+. I’m not sure about other carriers and I’m guessing most just check if they are able to support the band in their country so in general the other model doesn’t work. But Airtel isn’t one of those companies.
-> In fact I proceeded to but the series 7 with cellular only after I found the solid proof it’s compatible after looking at the Airtel site.
So, if I were you, I would get SE with cellular