The article is not very clear, but my interpretation is that this is about the e-mails sent from Proton Mail, not users being able to access the Proton Mail web site.

A VPN won’t help you if the server for the recipient of the e-mail drops the e-mail.

So, basically imagine that all Internet service providers in India have to block any e-mail from @proton.me and not deliver them. I think that’s the idea.