unfortunately cloud computing and marketing have conflated reliability, availability and fault tolerance so it's hard to give you a definition everyone would agree to, but in general I'd say reliability is referring to your ability to use the system without errors or significant decreases in throughput, such that it's not usable for the stated purpose.

in other words, reliability is that it does what you expect it to. GCP does not have any particular guarantees around being able to spin up VMs fast, so its inability to do so wouldn't make it unreliable. it would be like me saying that you're unreliable for not doing something when you never said you were going to.

if this were comparing Lambda vs Cloud Functions, who both have stated SLAs around cold start times, and there were significant discrepancies, sure.