The Spurs could not have offered Leonard a supermax deal until July 16, 2018. There are rules about it, including when an offer can be made. (The significance of the July 16 date is that Leonard signed his last contract with the Spurs on July 16, 2015, and certain possibilities are annualized.)
Leonard was traded on July 18, 2018. It should be obvious, in order to achieve that trade, serious talks on it must have been underway before July 16. There's no chance the lawyers can get all that contract crap reviewed and expressed - for four players and a draft pick and a cash transfer - in only 24 hours. Lawyers get paid by the hour, btw.
Did the Spurs offer Leonard the supermax?
No.
During that very brief window of opportunity when the Spurs might have made the offer, on the 16th and 17th, the Spurs, and the Raptors, already had their lawyers hard at work on the trade. They must have.
The Spurs had already decided a supermax offer was pointless. He was leaving.