I'm not well versed into these things, but I think people are conflating 2 things to be the same. I think that if you'd do the entire process through Google, you have to first pay and get the device/firmware certified, and then pay to use Google's services to send the firmware to devices. There are separate costs here for Google, one to pay people examining the device to certify it, and one for server costs to host and upload the firmware from.

Seems that F(x)tec chose to do the first thing via Google, in order to get support for Google Pay and the like. But chose to do the second thing via a third party, probably because it's cheaper than Google but doesn't have any direct negatives like no longer having Google Pay support.

So, just because they got Google's certification doesn't mean they can just change to a different OTA provider. They probably could, but whether they have Google's certification isn't really relevant to this I think. It would come down to if F(x)tec thinks it's better to use a different OTA provider, and if they can afford it. But again, I'm not that well versed in this either.