Doesn't really answer the question. There is no way a player moved like that by simply playing games and having good results; one does not miss out a whole tier!! So, either WGT did something or it is an error. If WGT did something, why are you afraid to say? And if WGT did do something, then at least we know steps are being taken to combat the sandbagging.
I get the impression WGT did something. Think about it, if it was an error the WGT reply would have been along the lines of "Thanks, we'll look into it." or "I passed this on to the devs to have a look at."
Maybe its one of the backend things to combat multi-accounters/sandbaggers MisterWGT referred to in his chat: "we are doing some things on the backend as well with our data analysis team."