This is just objectively miles from the truth.
I don't know whether people have some bizarrely inflated opinion of Stanton, or recency bias against Glennon based on how poorly he played for the Bears, but the numbers don't lie (and even if you think they don't tell the whole story... they're not even close).
Glennon has a career 83 passer rating (770 attempts) with 34 TDs and 20 INTs. In his awful season with the Bears last year, he still had a 77 rating with 4 TDs and 5 INTs. Those are better than average backup numbers.
Stanton has an appalling career rating of 66, with 20 TDs and 24 INTs on 659 attempts. The best stretch of his career, when he managed to go 5-3 with the Cardinals, he had a 79 rating, on par with Glennon's "abysmal" season with the Bears. The four seasons before that one, his rating averaged 63, and the three seasons after, he averaged a truly putrid 56.
Stanton is really, really bad. Glennon is fine as a backup - certainly nothing to get excited about, nor to be distraught about.
...dbs