When I began reading this thread I thought roughly the same way. But not for long: if plugging together parts is building yourself a computer, then hosting on lower-level services is self-hosting.

But then on the other hand I'm on the "that's not self-hosting!" side when you shift from your own needs to selling a service: if you run and sell a gitsomething.com and it's all on AWS, that certainly isn't self-hosting. And again the similarity holds: when you operate a computer brand and it's all off the shelf parts you're not really a computer builder, even if the exact same configuration would nicely run under "I built this" when some by a consumer.