I think the point is that if anyone of any significance was using Mastodon in this context like they are Twitter, they would just block that too.

So it's not as if Mastodon offers anything different/better in this case.

I think it's harder to block Mastodon because there are many instances. You can block the main ones, but people can still use smaller ones to access content from all Mastodon instances.

Twitter has one domain and probably a known IP range. Mastodon has (at least) hundreds of domains and IPs.