[0xJRS]

I think they assumed that the reader would know 5.17 and (kernel.org) next to it would mean the Linux kernel

[dr_zoidberg]

Well it's the subject of the email sans the "[GIT PULL]" at the start. Maybe they just went with it for the title? It's been changed to "...for Linux 5.17" so it's clear now for those just reading the headlines.

[nicce]

Yeah, Linux is the name of the kernel, not some distribution.