It also references Section 8.3 of the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3A, which also says IRET is a serializing instruction.

> Interrupt processing (returning from an interrupt handler, actually) is fully serialising on x86, and on other platforms, no doubt: any userspace instruction either fully executes before the interrupt, or is (re-)executed from scratch some time after the return back to userspace.

Approaching this from the perspective of not knowing that iret is serializing, this quote doesn't really provide a good reason for why it should be serializing. You've mentioned the manual, which is probably what should've been quoted, because the thing I extracted out goes on to say that it must be true on other platforms too when it evidently is not. The rest of my comment was a hypothetical on how you might observe if iret was non-serializing, which (given what the manual says) must be unobservable.