> or if it's some address that's truly free and could be picked up by another process

This isn’t a thing; virtual memory (https://en.wikipedia.org/wiki/Virtual_memory) is very old, and Linux has never not used it. It takes a separate vulnerability (a “local privilege escalation”) to do what you’re getting at. (However, those are plentiful, and arbitrary malicious code with PHP process privileges is often already catastrophic for a web service.)

That said,

> OP seems to be implying that there's no way the UAF bug could be used to run arbitrary code with higher privileges than PHP itself

No, nowhere does the article imply that at all.