Yup, and that's kinda intentional. The design is to be a monitoring system, not a generic TSDB. What it does needs to be simple, fast, and reliable so that your alerts get sent out.

The original design inspiration, borgmon, also was a terrible storage system and had an external long-term store layered on top of it.

This isn't a design flaw, it's an intentional trade off to make the core use case as bulletproof as it can be. Having seen "monitoring systems" based on something like Cassandra, aka distributed storage, is cringe inducing. The first thing to crash a the first sign of network trouble is distributed storage.