Horford will sign elsewhere long before July 12. Boston is already prepared to give him a max offer on his visit according to reports.
If Horford wants to go to OKC and they have a tentative deal set up to clear cap space, why wouldn't he be willing to wait? Players waiting for their desired team to clear cap space is fairly common, and it's also fairly common for a marquee FA to not sign with a team until closer to mid-July.