A Black Adam solo film probably the best move they were able to make right now. The Rock is box office gold right now, and he has been more than patient with WB while they ignored Shazam for what feels like an eternity now while they greenlit any number of other projects, despite having signed The Rock up ages ago.
Say what you will about the DCEU so far, but the complete and utter failure to do anything with The Rock after sitting on a project he's been incredibly hyped about for quite some time now would be the crowning achievement in a catalogue of already pretty sizeable fuck ups.
I thought it was mostly because he was so busy... I mean dude filmed what 4-5 movies last year?
This year he seems to have a lighter schedule so probably pushed for them to get the wheels in motion.
Wouldn't be surprised if started filming before year's end.