Yes, now other online platforms have to deal with LLM spam as well. That doesn't really mean we have a solution for that.
Social media hasn't been able to keep up with spam even before LLMs became this big. With LLMs, mass-generating legitimate sounding spam became cheap and effortless.
I suspect the solution will look something like "If you make too many PRs that don't get accepted, it flags your account for manual review... Or just preemptively cuts the account's authority to propose PRs to other people's projects pending appeal." As far as I'm aware, nobody's really drowning at Meta or Reddit regarding LLMs; they've been dealing with low-quality human-generated content at volume for decades. Perhaps this is just a new challenge for GitHub specifically?