Given the fact that the change which occurred is an "apples to oranges comparison" change (PostgreSQL and an SQL normalized DB table structure to MySQL and a noSQL style single table key-value store) then there is some credence one can put towards the rumor that some (or most) of the change may have come about from a PHB [1] saying "do this, this way".
[1] PHB (Pointy Haired Boss, Dilbert cartoon reference)
(note I said build, not use, different skillsets between driving a car and re-configuring the engine to run on seed oil)
Honestly, why would they? If there is a product that does what they need, why spend resources improving another.
Just because it's open source and they could spend money improving it, doesn't mean a company will spend that money improving it.
I would hardly say MySQL does what they need without any improvements. They built an entire second platform on top of it.
also their replication strategy looked like a joke and not enough automation. hopefully they never use galera, else their bad engeering practices could actually suffer in huge data losses.
I've once run galera-cluster and it pretty easy came to data losses especially after short network splits which occured randomly at the network.