[michaelt]

PTP is a standard feature on a lot of Intel chipsets so a lot of servers have it 'for free' and aren't using it.

It timestamps packets at the network card - independently of any kernel timing.

[zinekeller]

Addenda: Intel networking chips, so even in AMD or ARM systems this actually works (assuming that you have the correct OS and drivers/modules/support software).

Also, this isn't exclusive to Intel: most modern (server) NICs at least support TCXO-based timings, and some built for precision uses OCXO for even better precision.