Having a stock operating system certified by google is usual/ordinary/expected. That is how pixel support worked for many years. This approach is borderline necessary as paying an ODM directly is very expensive and loses out on several benefits.

Motorola stands to make a lot of money from this. They need to improve hardware but they benefit from that hardware. They likely provide early access but they already have that access themselves. From Motorolas perspective, they take on very little risk, if any, to tap into a market that has since been ignored, for existing GrapheneOS users, prospective GrapheneOS users, and for alternative OS users that benefit from yellowboot support. This is the most ordinary approach that could have been taken.