It looks like the 25% max is guaranteed, the "up to $250MM" might be escalator clauses built in, because 5/239 is the current Rookie 25% Max extension (and that is what is reported on Spotrac). So Chet will need to make All NBA, win MVP or DPOY this year for that to escalate up to $250MM.
But even with it only at 5/239, this deal still takes him over 25% because of the 7% salary cap increase next year, and Spotrac is only forecasting 5% increases to the cap after that.
All of a sudden, when we thought that deals with max bird raises would mean they were declining deals as a % of the cap, are actually increasing deals. This is going to cause a major shift in how teams approach things (putting this in bold because I think it's important)