I slept right thru that storm but yes AAL needs the airplane and likely the crew as well in STL to cover the downline flying on it, if they can also carry the PAX that's an added bonus.
I'm only assuming they carried the PAX back to STL some 90 minutes later. There is no obvious way to tell.
It's surely dependent on duty time as well so the flight crew would have the 90 minute layover plus the 45 minute flight plus whatever cleanup time is required after a flight for a flight crew to close out a flight.
I saw the same thing happening again last night. I watched 3 different flights, one from PHX, 1 from LAX, and 1 from BJX all fly holds near KMDW as an ugly storm was brewing. All of them eventually diverted to KSTL.
I didn't look to see if they later went on up to MDW as in the case last night, the WX continued on a southerly track for the rest of the night.