Well, I cannot directly comment on Thibs' recommendations as I have neither tried those doses myself nor have other experience of knowing specific cases.
However, first, the recommendation for that much glutamine was not in general, but for those that are choosing to not consume carbs as an alternate way of making up for this lack.
Second, the thought that one can assume necessarily, without information, that a given thing won't do anything at some higher than familiar dose, or won't give improvement, is not really the right way. For example I was skeptical of BCAA's, as they have been around forever and at doses that were typical back in the day, such as 5 grams, nothing important happens (IMO.) But when Tim told me, no, the thing is much higher quantities such as 20 grams, I would have been making a mistake to reflexively assume that that must do nothing or be too much. (And no I didn't assume that, and yes Tim was right.)
That sounds like a lot of glycine to me but I have no experience with that much so I have no basis to say anything against it. Thibs is a smart guy and has opportunities to see what works and what doesn't work with a number of people and so I would not, myself, assume him wrong.