Care to explain or provide any source for this?

There are ways to send iMessages programmatically. Apple does check for spam, but it’s not foolproof. And of course, it won’t help you against a targeted attack.

I can count the total automated iMessage spams I've received on one hand. I can't do that with automated SMS spam I received in the last 24 hours.

So yes, not foolproof.

How exactly does any of this prevent people from sending spam to you?

If there are ways, please share any documentation you have on it. I haven't found anything useful, myself.

I didn’t research it that much, but a good place to start would be AirMessage [1] or Mautrix [2]. Both of these require a Mac to work – it might work on a Hackintosh, though, or maybe using same tricks those forks of OpenHaystack use to run without a Mac (no pointers here, sorry). Hope this helps!

[1]: https://github.com/airmessage/airmessage-server

[2]: https://github.com/mautrix/imessage

Thank you.