The Bears’ quarterback has been better of late, but not that much better.
*’of late‘ means recently.
Point 78 in ADJECTIVES/comparatives is defined as:
‘not that much’ to modify comparative adjectives to a small degree
1 NOT THAT MUCH BETTER 280
2 NOT THAT MUCH BIGGER 109
It’s just not that much bigger.
(listen to a similar sentence)
3 NOT THAT MUCH HIGHER 98