Despite what the Amazon marketing is telling, Redshift is not really a "fork" of Postgres.
To my knowledge they only used the SQL parser and the wire protocol from Postgres.
The optimizer, query executor and storage engine are totally different. The whole "Redshift is Postgres" is complete marketing BS in my opinion.
Anyway, does it really matter? What is someone looking for a fast 'postgres' for analytics actually interested in?
(I didn't realise this was just an extension - in which case I'm amazed it's possible, but that obviously makes it an easy sell if you're already running pg. But if you're shopping about for managed solutions (which is obviously what Hydra wants to sell) with 'postgres' criterium, you're interested in the query language and maybe the wire protocol, surely?)
Many Postgres features aren't supported on Redshift (set returning functions, indexes, ...) and many tools that work just fine with Postgres error out because Redshift does things differently or doesn't support features that Postgres does.