I've been vocal in the past about using an OSI-approved license for one very specific practical reason: it gets you the ability to be packaged for Linux distros etc. that have policies that match the OSI definition (usually by having terms that match the OSI's current definition, not by delegating the decision to the OSI). In my opinion, that's important for adoption - not as important as it used to be in years past, yes, but still important. Even if production users are going to want to run the latest version (or run a commercially-supported binary), the ability to apt-get install something on your laptop to test it has value, as does the ability for other pieces of software in a Linux distro to depend on it.

That said, given that they're following a more-or-less "open core" model where there's a clearly FOSS portion (Apache 2) and some non-FOSS extra features on top, I don't think that this is going to be a practical barrier. It looks like the Apache 2 portion of Timescale is substantial enough that you can package it in Linux distros and use it. So in this particular case, I think it doesn't matter whether the rest of it is technically "open source" - it very clearly isn't going to be accepted by Linux distros, and it very clearly is fine for its intended users, and both of those are okay.