You’re right that they’ve included his $5M salary for 2019 in the dead money.
But it doesn’t change the analysis directionally. If he’s on the roster March 15 his cap hit will be $9.2M (comprised of the $5M base + $4.2 of prorated signing bonus) If he’s cut today they won’t incur that cap hit but the full remaining 4 years of prorated signing bonus, or $16.8M, accelerates onto the cap.
So cutting him today would result in a net cap loss of $7.6M (16.8 - 9.2) They can designate him a June 1 cut, such that $8.4M (half of the remaining prorated signing bonus of $16.8M) accelerates onto the cap this year and another $8.4M accelerates onto the cap next year.
That means they would have a cap savings this year of $0.8M ($8.4M against the cap as opposed to $9.2M if he were kept) but a net cap loss next year of $8.4M. Combined the net cap loss over the two years is still $7.6M. It’s just spread out differently.
The cap economics still dictate that he WILL NOT be cut before March 15. They’d be wasting $7.6M of cap space, however they choose to spread it.
Next year’s cap hit and dead money are irrelevant if he’s cut today. The contract ceases immediately, the remaining prorated signing bonus accelerates immediately (though it can be spread over two years by designating it a June 1 cut) and you back out this year’s cap hit (which you don’t incur since you cut him) in order to determine the net cap hit. Again, in this case it would be a net cap loss of $7.6M.