You're looking at average air yards there, not average yards. His AY/A was 9.7, but his Y/A was 8.7. He definitely looked deeper this year, especially considering that his AY/A were 7.3, 6.3, 6.3, 6.8 prior to the 9.7 jump. The Herbert stat you looked at for 8.1 is actually 9.0 when comparing the same stat. Herbert's numbers are consistently higher than Pickett's despite Herbert playing in a system where he didn't often push the ball deep downfield.
The hand size isn't an advanced metric, but it is another ding against him. The hand size concern correlates to his propensity to fumble, which he averaged once every other game. During the same time span in the pros, Russell Wilson averaged a fumble once every four games (and he's got 10.25" hands). This is a much different issue than arm length, which can be compensated for with quick feet.
Some more metrics, basic or advanced, that hurt him:
While he's 13-10 vs. top-25 opponents in-season, he's only 3-7 against teams that remained ranked at the end of the season
His combined stat line from those 10 games is: 183/302 (60.6%) for 1,923yds, 8 TDs, 8 INTs, 6.0 Y/A; 92 rushing attempts for 120 yards (1.1yd avg), 3 rushing TDs, 5 fumbles. That averages out to a really bad stat line on a per game basis.
Before 2021, he was 2-6 with the following stat line: 138/230 (60%) for 1,368yds, 4 TDs, 8 INTs, 5.5 Y/A, 79 rushing attempts for 85yds (0.6avg), 2 rushing TDs, 4 fumbles