This idea that any decision on Lamar Jackson is up to Bill, is the same exact reason Kraft picked Bill over Brady. Kraft doesn't want to be seen as a cheap owner, so he keeps Bill around to take the blame for keeping cost down. For all Brady did, keeping him wouldn't really effect the market value of the team at this point.
Honestly, if Kraft wanted to pay Brady the market rate, Brady would have never left. A contract of Lamar Jackson's caliber is not a GM call, but that of the owner. If Kraft tells Bill, I want Lamar, do you really think Bill would say no?