There are multiple ways they could go. With the void years already on there, they could give him a four-year deal and it would actually be more like seven years as far as parsing the money. Or they could just replace the void years with contract years. For some reason, they want to disperse the money, which would seem to indicate if nothing else, that they want to get a long-term deal done. If they were only wanting to pay him this year and then let his contract run out and let him leave, it doesn't seem like they would have any reason to add the void years.
I think what makes the most sense is for them to keep them on any contract they give him because then they can push any bonuses out and lower the initial hit. I mean, $240 million (counting the money they'll still owe him) over eight years is a lot easier to absorb than $200 million over four. In the end, they still would have to take the final hit over two years at the most, but it would make Years 1-4 much more palatable.
I do agree that they could have just added more void years at the time of the extension, but maybe they thought it would be easier to get him to agree to the void years now, so when they do get the extension done, they can sell it to him as a 4-year, $240 million deal instead of a 6-year, $240 million deal (with the extra two void years added at that time).
I don't know for sure, other than the fact that adding more void years doesn't seem like something a team would do with a player they consider to be on a one-and-done deal.