Nothing of that has to do with OEMs being forbidden from shipping QPRs though.

Graphene isn't OEM so they don't get access to Android repository before AOSP drops. OEMs - like Motorola for example - do have access to partner repository and can ship QPRs if they wish to do so. Most often OEMs do not want to push updates (and it took years to make them push regular major Android updates).

> I think they know what they are taking about.

Err, if you look at that account you'll find that the person running it has a penchant for overexaggeration and selective truth telling when they feel aggrieved.