The team has to spend 90% of the salary cap (the Sterling Rule). For the next three years, the Hawks (even if they get Horford) will be way under. They have to spend that money. You might as well get something for it.
They will have zero veteran leadership on their squad next year. That is a problem for a young team. In essence, they would be spending $10M/year of the $40M+ they MUST spend.
BTW, Horford is only guaranteed $14.5 in 22-23 unless his team makes the NBA Finals. That's $5.7M less than Capela is guaranteed.
According Hoopshype, barring Horford's team winning an NBA championship, he is guaranteed $69M over the next three years. Capela is guaranteed $57.3M. Actually, you were closer with your $10M total versus my $10M/year.
"Al Horford: Only $14,500,000 guaranteed in 2022/23, HoopsHype has learned. He would get $19,500,000 guaranteed if his team makes it to the NBA Finals in 2019/20, 2020/21 or 2021/22. His 2022/23 would be fully guaranteed if his team wins the NBA championship."
They can also stretch that $14.5M if Horford isn't worth having.