The Switch is a portable, so it makes sense to include GBA. But in any case, as I said above, the release timing will mean launch-aligned comparisons flip-flop from "nearly the best ever" to "around fifth all-time". Calendar-aligned monthly comparisons are thus probably more informative. By those, Switch is doing very well, but not "easily hit 2.5m in December, no sweat!" well.
I agree with that. 2.5m is a huge number and will depend on a lot of factors. It is faar from a lock.
However, I believe that the system should do 2m+ given its first December numbers (1.5m), its Holidays line-up (Smash & Pokemon + Mario Party) and the bundles it'll have (Fortnite, Pokemon, Smash).