That's the thing, Howell doesn't make it improvising alone. Russ did, while Howell can do it he did alot from the pocket more than Russ in his early years. To me there is no comparison for the 2.
Howell was mostly improv at Washington. He played like a hybrid between Ben Roethlisberger and Russell Wilson.
He held the ball longer than any other QB too and tended to avoid the middle of the field just like Wilson.
He was not great as a pure pocket passer, very clunky there.
Also, if you notice the current NFL trends it is shifting away from the trends of the 2010s.
In this era we saw guys with mediocre passing fundamentals and elite athleticism shine. With modern day defenses those guys aren’t as successful.
Gone are the days when you could run simple deep crossing routes to beat the cover 3 everyone was rolling out. Players are having to be very savvy presnap and post snap, they have to go through their progressions and play fundamentally sound football.
Howell is not that guy yet, which is why he’s not starting. He’s unrefined and very sloppy as a pure passer. His deep ball also has a huge wind up to it. His footwork is sloppy as well.
As a backup he’s great, but if he’s in a starting position I guarantee we would have 1-2 less wins at the moment.