mmmm I understand you want to talk about it.
But I find GrapheneOS is usually quite transparent with things. So my understanding is if they chose to announce they have an OEM but not who, there's probably good reason for it.
Here's a quote from the project account:
GrapheneOS The terms with our OEM partner are not really under NDA, and neither is which OEM we're working with, but we don't want to publish which OEM we're working with until an official announcement with them. We'd also prefer if they make the announcement and we spread it because it would look better.
That is why I personally prefer not to speculate about it. It's probably best we wait it out until the project or the OEM releases any further information.