I found this curious as well. Initially, it wouldn't surprise me if some GMs are poor at evaluating the G League performance of 18 year olds. However, as analytic-focused as front offices are these days, I can't imagine that would last long. To me, the argument is not all that different from saying Wendell should have gone to Harvard instead of risking the chance of exposing his weaknesses at Duke.
In the end, it is in the NBA's interest to have top prospects playing against the best possible competition. If the best competition is in the G League, I could even imagine a not too distant scenario where top prospects are strongly encouraged to play in the G League if they don't want to risk their draft status by raising questions about the level of competition they faced in college.