Brady definitely wasn't playing like the GOAT in 2019. I know he didn't have a ton to work with that year, but he also just wasn't that great.
Raw data
2016: 67.4%, 8.2 y/a, 28 td, 2 int, 112.2 rating
2017: 66.3%, 7.9 y/a, 32 td, 8 int, 102.8 rating
2018: 65.8%, 7.6 y/a, 29 td, 11 int, 97.7 rating
2019: 60.8%, 6.6 y/a, 24 td, 8 int, 88.0 rating
Brady's last 9 games of 2018 through all of 2019, plus the four playoff games in those two years:
Last 9 games of 2018: 205-320 (64.1%), 2,479 yds, 13 td, 4 int, 96.1 rating
16 games in 2019: 373-613 (60.8%), 4,057 yds, 24 td, 8 int, 88.0 rating
4 playoff games: 105-162 (64.8%), 1,162 yds, 2 td, 4 int, 79.8 rating
TOTAL (29 games): 683-1,095 (62.4%), 7,698 yds, 39 td, 16 int, 89.1 rating
That's not a small sample. We see consistent decline from 2016-2019. The last 29 games of his career, the numbers are actually pretty ugly for him. Clearly the data doesn't suggest that he was a guy who would be lighting the NFL up at age 43 or 44. Now he made some huge plays in the Chiefs AFCCG, and had that one great drive in the Rams Super Bowl, but he also made some key mistakes. He wasn't good in the Tennessee playoff game.
Again, he didn't have the best weapons around him, but an objective look at his performance suggests decline. Not "wow this is a guy who will be playing at an MVP level at ages 43 and 44".
That Brady has gotten back to this level is astounding and a testimony to his greatness. But it's totally understandable to me how a guy like BB could see this and think, "yeah I'll take him but not at the kind of money it will require because I don't know that he's ever going to be TOM BRADY again."