They can't offer much. They would need to trade someone (Millsap) if they want to resign Horford.
You're wrong in the context of "what they can offer".
This isn't about how much they can offer, it's about ability to sign Howard and Bazemore at all while Horford still has a cap hold.
They do need to make some trade though, but not as big as Millsap. Something like Splitter and misc should give enough cap space to fit Howard, Millsap, Bazemore, and give Horford whatever he wants.