The proof silver dollar is limited to 500K. The 3 coin set is limited to 75K. The dimes are the coins that could possibly be offered in some other kind of way, exceeding the 75K.
It says they will not be offered in any other set That got me thinking, what if there was a sudden mad rush for the proof or UNC dollars and they were only able to crank out like 10,000 sets..... That'd be crazy, right?!?
The total mintages of the series is set by Congress but they do not specify mintages for the different set options the Mint offers. So in theory the Mint could change the mintage of the set. However since they have already announced the maximum mintage, and that the dimes would only be available in this set, they would be setting themselves up for a lot of trouble to suddenly make changes now.
The mint created a firestorm with the reverse proof Silver Eagle Set a few years back. I don't think they'll pull a stunt like that again.