[cdesai]

How can they do that with HTTPS though?

HTTP sure they can happily MITM and redirect, but with HTTPS you need a valid cert.

[aquatica]

Your browser shows a TLS warning saying that the certificate doesn't match. If you override it, it brings you to that page.

This is not a DNS block, the IPs are owned by Google, Italian ISPs are forced to forward the traffic of a blocked IP to AGCOM's servers.

[toyg]

DNS.

[gruez]

Connecting with https prevents any DNS poisoning, unless the ISP managed to get a fraudulently issued certificate or a MITM root CA installed on the end users' devices. Neither seem likely.

[toyg]

It's not poisoning, it's blocking.