Now seems a good time to point out, the wheel has literally been reinvented over and over again. The wheels of yesterday were terrible. Each version gets better. It's fine, reinvent away folks :)
A company like figma (billions in revenue) putting a small team to implementing a database sharding solution for an un-implemented use case (RDS, not just postgres). AND open sourcing it creating a value for the community is a net-good for the industry.
Building something super simple can be ok for the base use case but if you are a multi-billion dollar company you can probably afford a few dbas to actually make your platform good.
As companies get larger they move further up the stack whether it's sharding techniques, databases, custom orchestration software, their own networking hardware, etc.
It works for Figma(for now), but for it to work as a solution for other companies with different hardware, data schema and access patterns will add even more complexity to the mix.
It's a excellent solution but I don't think it be good enough in the long run.