I think GrapheneOS is caught between a rock and a hard place, and I think whatever decision they make is going to have huge implications for the project's long-term viability.
On the one hand, Pixel 11 omitting MTE is ridiculous. It was an opportunity for Google to improve the security of its users, security which is now compromised. Yes, they weren't using it on the stock OS, but frankly, they should have implemented it more liberally. Even if Google doesn't care about your privacy, they definitely should be caring about your security and it's not only a bad look, but a real risk to their users when they compromise on a feature like this.
On the other hand, the Motorola partnership is far from guaranteed. Yes, maybe for the first few years it will be great, but anything could happen afterwards and I must say that the track record for companies working with custom ROM devs (in particular Cyanogen with multiple OEMs) has not been great. Like it or not, GrapheneOS is a niche project, and companies aren't incentivized to support niche projects unless it's for something like a government contract or enterprise. It is a huge risk to the project to rest their entire future on Motorola when Motorola could easily end collaboration for future devices in a few years (not to mention, be banned from the US if political tensions with China continue to rise).
Personally, I think GrapheneOS needs to diversify who they're working with and continue development for the Pixel 11. Yes, it's far from ideal, and it would involve a glaring compromise in security. But who's to say that Motorola is going to keep playing ball in a few years? We have yet to see Motorola establish a real track record of working with GrapheneOS, and I think going all-in on this partnership is risky when it has yet to prove itself in the real world. We don't even have a Motorola device that can run GrapheneOS yet!
The way I see it, it's best to provide harm reduction to users and diversify partnerships, and support the Pixel 11. GrapheneOS could state that they strongly recommend Motorola devices and lay out reasons why, but leave the Pixel 11 (and future Pixels) as an option. Samsung won't allow bootloader unlocking in the US and hasn't for years, I wouldn't touch Nothing with a 39-and-a-half-foot pole after the Sunbird incident, leaving Apple and Google as the only real options here besides Motorola.