The video is very impressive, and it would be good for the NFL -- and obviously the Bengals -- to have Carson back for the start of the season.
But temper your enthusiasm by remembering that there's a huge difference between controlled drills and actual game conditions when 11 large, strong and ill-tempered men are focused on hitting you as hard as they can. Jerry Rice returned from a torn ACL and MCL in 1997 after 4 months of rehab to play in Week 16 or 17 after injuring his knee in Week 1. In his first game back, his kneecap shattered on impact with the ground. Specualation is that the joint wasn't ready for the stress put on it due to the trauma of the injury and rehabilitation.
Medical science continues to progress, but the human body still has limitations. And the wear and tear of the NFL on QBs is already substantial, let alone when the QB in question starts out with a seriously injured knee.
Finally, from the FF perspective, there's a lot more to it than just will he start in Week 1? Will he stay healthy all season? What level of production will he achieve compared with the 2005 numbers in Cincinnati? Will the coaching staff emphasize the run more, especially early in the season? No one knows for sure.
On the other hand, there's risk at every position, and this is an opportunity to get a top 5 QB at least one or two rounds later than normal. For me (in redraft leagues), it will be a question of when to draft Palmer. At what point is the risk worth the reward and worth the opportunity cost of drafting someone else with theoretically lower risk.