Last year, Bill O'Brien ran a hybrid pro-style / spread offense, which was much more heavily tilted towards pro-style. Heck, when you have Bryce Young as your starting QB, you can run just about anything... Young was exceptionally good at staying in the pocket, reading receivers, and timing his throws just right.
Milroe simply played as a true dual threat QB in a spread manner, where he'd look for the first option in the passing game, and if that were covered, then look for the second option. If the second option weren't open, he take off running, unless the the third or fourth option were obviously wide open. Basically, Bill O'Brien's package for him was similar to how Jalen Hurts would run the offense. He actually looked pretty comfortable running that simplified offense.
During this year's spring game, he looked confused out there, trying to read the receivers, even though he had "scanned" the field earlier.
Now, to be fair, Alabama has a top notch defense that would make life miserable for almost any opposing offense. Still, it's a spring game, where players aren't going to be going all out.