Timescale looks like an Interesting project, with most of it under the Apache License. However, it looks like some of their key value-add enhancements for PostgreSQL Columnar Store are under a different license with some very specific prohibitions about some types of commercial use:
"2.2 Prohibitions. Notwithstanding any other provision in this TSL Agreement, You are prohibited from (i) using any TSL Licensed Software to provide time-sharing services or database-as-a-service services, or to provide any form of software-as-a-service or service offering in which the TSL Licensed Software is offered or made available to third parties to provide time-series database functions or operations..."
https://github.com/timescale/timescaledb/blob/master/tsl/LICENSE-TIMESCALE
Firstly, that restriction does not meet my definition of open source.
Secondly, the modest amount of value add they provide to existing columnar store in PostgreSQL is respectable, but hardly secret sauce.
They are entitled to handle their code enhancements as they see fit, but I think it borders on them abusing opensource software with a product that exploits PostgreSQL doing much of the heavy lifting, with them adding a modest amount of enhancements and then locking down the result. Me personally, I just don't have much respect for that.
All standard disclaimers apply, and then some.