Because it's a terrible blog post. If you applied this criticism to any other author, it would be valid.
But because it's pg it's different? No, it's still a bad post. There are a plethora of other reasons Twitter was a big deal. It being a "protocol" wasn't one of them.
I don't think the post has held up, but the "it's not a protocol, it's just HTTP on top of TCP/IP" is a lame argument. It's clearly a protocol. I've been doing protocol engineering work since the mid-1990s, and people have been saying things built on top of HTTP aren't "protocols" since HTTP went mainstream. I was one of them, in the 1990s! That was dumb of me; most new important protocols since then have been built on top of HTTP, and I expect that to continue.
The subtext of these "it's not even a protocol" arguments are that Paul Graham doesn't know what a protocol is, which is not a plausible argument. Why make it?
But because it's pg it's different? No, it's still a bad post. There are a plethora of other reasons Twitter was a big deal. It being a "protocol" wasn't one of them.