The left seat turboprop to left seat jet can be done. Jet2 did it, transferring guys from the F27 to the B737 and nearly all were fine. Globespan, were taking on Loganair, Saab 340 pilots and having them fly 737NG's to Canada. So, I'm pretty sure many of the Electra and ATP pilots could go direct left seat.......doesn't sound like that is what is happening though.
The ones I have seen struggle seem to fit a type. Older guy, flown X type of aircraft for many years, gash habits and come into the type conversion thinking they are God's gift. There were guys that I worked with at AA like that, but surely there are some ATP and L188 guys able to make the move?
Sounds like exciting things are happening are happening at AA, just not for many of those that have been there for a while and deserve the break.