This does not require a state-of-the-art LLM. You could easily catch this kind of thing with 1990s-era technology.
Some possibilities:
Carriers and device makers are required to deliver texts? (No, the opposite is true. There's a law that they have to block anything that is reasonably determined to be spam)
The carrier requires Apple to not take additional action on messages it delivers? This would be surprising to me, and Apple holds a lot more power than the carrier does.
99.9% of such messages are blocked, and I see the 0.01% that get through? Possible.
Apple just doesn't care? I doubt it. Apple obsesses over so many other parts of the user experience (eg Visual Voicemail, which reduced the amount of time I spent dealing with voicemail spam by 90%) that I assume they'd pay attention here too.
I would love for each receiver's press of the "Report Junk" button press to cost senders 50 cents. I think that would deal with the problem very quickly.