Earlier quoted context omitted.
The info "is https available" is not secured either. The ISP can just block any packet on port 443 and force http that way. It would break links but wouldn't break people entering the address via the URL bar. A real improvement in security would be Google caching the data, and either offering it via a custom API or just signing it and appending it to their 8.8.8.8 DNS responses. Per default, Chrome already sends the…
HSTS preload lists exist. And the scenario where an ISP blocks https connections is unrealistic, the server can simply refuse to serve content on http other than redirects.
And it doesn't matter whether the legitimate server is refusing to serve plaintext HTTP if you're not talking to the legitimate server in the first place. The attacker can serve whatever they want.