The point being that the AGPL licensing of RethinkDB caused problems selling SMC to customers. So why invite those same problems again by replacing it with other GPL'ed software?
> All the code I wrote related to this blog post is – ironically – AGPL. Basically it is everything that starts with postgres- here.
All the code they just wrote for the Pgsql rewrite is AGPL. Obviously they can relicense since they own the copyright, but it's weird to mention MySQL's GPL license in passing as a reason they didn't went that way yet put your own code up as AGPL.
The GPL license essentially triggers on distribution, but if you have some SaaS app it's not clear that this falls under distributing your code since you're accessing what the code produces over a network. This is usually known as the "Application Service Provider" hole in the GPL.
With the AGPL that hole is closed and you have to be able to receive the source code for that app too.
This is all explained right here: https://www.gnu.org/licenses/licenses.html#AGPL