This is not true, Claude and ChatGPT have sent traffic to sites I maintain without any user-agent, it might be they have stopped doing this.

Google has done various versions of this as well, Grok too.

It could be these bots were in some custom harness or something, but they literally sent requests in very weird ways, that humans won't be doing.

I have ended up putting my sites behind vpns or anubis and similar. This is beyond painful.