He has a huge frame and pretty good athleticism considering kids that tall usually don't move so well. I'm guessing Brohm thinks he can fix his mechanics and maybe his football IQ is good enough where he sees more pros then cons.
Historically lots of SoFlA qb's labeled QB2's have outperformed that ranking: Tyler Huntley, Jordan Travis immediately come to my mind. If he checks enough boxes can't hurt to take a flier.