I seen some questioning of the legitimacy of this email.... apparently it had to be after the initial leak.
But assuming that last part is true, and Marvel is doing this through 2020, where they then have the option to pay $2B to get the rights back in full, then it makes sense why Sony is quick to push out all these spidey-less movies, such as Venom, Silver & Black, Kraven, as they too are on a time limit to milk the franchise, which is also why they don't care that Spidey ain't in them.
how they they they get to $12B in profit in that time, I have no idea, I'm not even sure how they get to $12B in gross, much less profits, but that's not my field of expertise.....
This could also be why Sony and Marvel technically have 3 pic solo Spidey deal, but supposedly the 3rd movie isn't guaranteed (Sony Option or something, iirc), and why there was only concrete plans for the first 2 movies.