Eh, I'm not stupid, so with that RTFM:
https://cartreatments.com/radiator-coolant-overflow-tank/
Pay attention to the end.....stuntnuts
"The coolant tank overflow is non-pressurized on almost all modern vehicles (
if the cap is plastic, it’s non-pressurized). That said, as coolant heats up it expands. So if you don’t have a tight seal, coolant can leak out and ultimately cause you to be low on coolant which would cause overheating. Check your coolant level and if you notice the level decreasing, get that cap replaced asap".
So, if it is plastic (it is) and if it needs a "pressure test" (so stated by the 'expert', but actually it is not?), how is it then it can be faulted?
Also, please show me where the "pressure relief valve on the overflow cap itself" exists.
Education is key, that is the purpose of these types of forums.
I know a lot, so if my questions seen stupid to some, excuse my ignorance, we all learn by asking questions.