The customers are quite isolated, but it doesn't mean that some services or errors do not propagate. In public cloud terms, think back on some AWS or Azure or even Gmail outages - you probably wouldn't even hear about them if it didn't affect millions of users at once, across security and availability boundaries.
I think it's easy to underestimate how many smaller outages there are in any period of time but which do not affect anyone or only a small number of people due to all the mitigations, due diligence that developers and SREs do, and the self-repairing nature of modern systems.
