It's pretty obvious to me that Stamkos at least wanted to hear what the Leafs had to say. He likely wasn't impressed with the numbers being discussed, and maybe position played into it as well (in years 2 onwards we need you to be a winger etc), and he decided to stay in Tampa. He probably told Tampa to hold on until this time back when the original offer was given.
If this is the case, this is prudent Leaf management, telling Stamkos if you are going to come, you are going to come on our terms, not yours.
I think Stamkos is cursing the Leafs winning the lottery, which in my mind changed the course of action for the Leafs...as it should have.