What does the proprietary thread pattern have to do with anything? a lot of custom actions have 18tpi threads..... just cuz you cant screw a remage barrel into it does mean much to a gunsmith. The tenon being a coned bolt face feeds better from the mag box. Just like the Terminus Zeus its a proprietary thread tenon because of the coned bolt face.
The GRB action is a great action to build off for the price, you wont come close to a Remington trued up with all the extras stuff that comes on a GRB, their whole rifles are another story, I wont go there. But look how many they sell a year, people are still buying them.