Maybe, maybe not. The theory is that in the hours those pilots would have had to have flown to get to the 1500 hours in order to be 121 qualified, they hopefully would have gotten the skills needed to keep them out of trouble. The issue is that 121 Airline flying is VERY routine and keeps the pilots well inside the performance envelope. Flight instructing, repetitive pattern work, flying into small airfields in mountainous terrain while VFR and things like that build skills in areas that you will, as long as everything goes well, never see nor need during 121 operations. Flying from Chicago to Denver or Charlotte to Montgomery on a normal day (and even during bad weather) doesn't teach a pilot very much about aerodynamics and if they haven't already gotten that knowledge (ex: you need to break a stall before you can fly out of it and, flaps can increase your lift so removing the lift when already very slow--and stalling--isn't a good idea) before they get in that sort of operations, they probably aren't going to get the knowledge there. 99.9% of the time that's fine and it won't matter, but for that .1% of the time when you need the knowledge and don't have it, people will die.