The Falcons would have taken on $65 million in dead salary cap money if they traded Ryan before June 1 (basically ADDING $38 million to their 2021 cap total). Obviously that was not an option and they could not have done that and kept under the salary cap.
If they traded him after June 1, they would have been able to save $2 million against the cap this year (but would still have had to eaten the same $65 million cap hit, just over 2021 and 2022 instead. They probably COULD have done that, but realistically, that also wasn't really an option.
Essentially, they would have been paying Ryan to play two years in another uniform and not getting any salary cap relief. So moving on from Ryan by trading him was never really in the cards. Some people suggested they should have drafted a QB and kept Ryan, but IMO, that would also be a less than ideal maneuver, as no matter what one of the team's best assets would be on the sideline for every offensive play.
If Atlanta really wanted to keep Jones, they could have converted some of Ryan's salary into a bonus (they could have done that with Julio too) . . . but that would kick the bad salary cap situation into future years and management said they were done doing that as the salary cap crisis would never go away operating like that.