I wouldn't be surprised if this is a move to have a large contract to move for Beal in a S&T next offseason. Smart and Horford would get close to what Beal's next deal would start at I think.
Assuming the Wizards move on from Beal, I bet they'll go into full rebuild mode. Why would a tanking/rebuilding team want Smart? In any case, we wouldn't need Smart's salary to make the deal work. Here are the numbers:
Beal's max will start at $40,525,450 (=35% of the cap).
Horford's expiring ($26,500,000) + Romeo ($5,634,257) + BOS 2022 first (let's say #16 once again: $3,311,040) would be more than enough salary to net us Beal via a sign and trade. Fwiw, we'd have to wait 30 days till after we sign our rookie in order to finalize the trade.