I disagree with the premise that global rating is any better of a proxy for true skill than TMM rating. Any format where the player is able to influence the circumstances of the games he is rated on is prone to have outliers.
In TMM you can, well, team up to achieve a higher rating. Team up with some lower-rated friends and the effect is increased.
In global you have even more influence on customizing the set of games you are rated on. Be a one-trick pony and only play Pass of valhallah. Or farm 800 avg rating lobbies to get your 2700 rating. Both very real examples of inflated rating on global.