I think this was one of the best trades I've seen in a while. Just as a recap, the net trade was:
MIA gives NO a 2019 2nd (pick 48) and a 2019 4th (pick 116)
MIA gives AZ a 2020 5th
NO gives MIA a 2019 6th (pick 202) and a 2020 2nd
AZ gives MIA Josh Rosen
If you take the median draft chart value of a 2020 2nd and 5th then do the math, MIA got Rosen for the equivalent of round 4 pick 102. That is an absolute steal for a player drafted 10th a year ago.
You are forgetting the discount for future picks. Your calculation essentially has Miami's 2019 second rounder cancel out New Orleans' 2020 second rounder.
Outside the top half of the first round, the rule of thumb discount is about a round or 50% of the points value. So a 2019 3rd is valued about the same as a 2020 4th. It seems like the discount isn't as steep today as it used to be, though.
So we got Rosen for roughly the equivalent of a mid third round pick, not a high 4th.
As an example of this discount, look at the deal we did with the Vikings 3 years ago.
Miami got 2016/#86
Vikings got 2016/#186, 2017/3rd, 2017/4th with the condition that the 4th would be the lowest Dolphins pick in that round (including comp picks).
In classic trade chart value:
Miami got 160 "2016 points".
Vikings got 16.6 2016 points plus 187.5 + 44 "2017 points" (median 3rd round value plus last pick of 4th)
This works out to a future discount of 38%.