It's all in the structure of the deal.
First, because it's a six-year deal, it averages $15 million per year.
Second, because it's a six-year deal, Elliott doesn't actually get paid $15 million in a year until his fourth year, when he would make $16.5 million if not restructured. The next year, Dallas can cut him and save $8 million in cap dollars, having paid him $6.3 million the first year, $10.9 the second, $13.7 the third and $16.5 the fourth. That would mean there's essentially $40 million of that $90 million that was funny money.
The problem/holdup with Dak's deal seems to be that he wants a lot of money on a shorter-term deal. When you have that, the money is more "honest" because you can't push dollars he'll never receive to the back end. What Dak needs to do is agree to a long-term deal that has impossible cap hits after four years and know that he's really signed a four-year deal because the Cowboys will either have to renegotiate it at that point or cut him. If he wants shorter than four years, that's a bit tougher to pull off because the Cowboys would have to get all his guarantees in over a shorter term, meaning they would have to pay him more now. Unless teams have reserved a huge chunk of cash to pay a major portion up front, that's not something that is palatable.