I think the disconnect is that promethus helps a user to shard things, but it's not automatic. Other time series databases and monitoring solutions automatically distribute and query across servers. It's like postgres vs newswl (aka foundationdb, spanner, etc.,).

While Prometheus supports sharding queries when a user sets it up, my understanding is that this has to be done manually, which is definitely less convenient. This is better than a hypothetical system that doesn't allow this at all, but still not the same as something that handles scaling magically.

Prometheus supports sharding queries the way a screwdriver supports turning multiple screws at once. You can design a system yourself that includes the screwdriver, which will turn all the screws, but there's nothing inherent to the screwdriver that helps you with this. If "scalability" just means "you can use it to design something new from scratch that scales" then the term is pretty meaningless.