I see Storm as a one card combo. Mainly because the entire strategy of the deck is focused on casting that one card. It's the same way that combo decks like Dragon/Trix used draw/search/tutor to obtain the correct pieces to complete the combo and win. Storm only spends time drawing/searching/gaining enough mana to go nuts with a lethal Tendrils/Brainfreeze. On this same idea Belcher can be grouped along as well. Since the deck is designed to run 1-2 lands and win on a single "belch" the only card that is really needed to win is the Charbelcher. You can have an all mana hand and a Belcher and just win undisrupted without any support cards what-so-ever.
I do understand that you obviously can't break it down into drawing a Tendrils is the win, because it does require 9 other support cards to generate a win, but on the other hand it doesn't require any specific card in conjunction with itself to be able to pull off the win. Belcher is an exeception to this, because you can technically topdeck a Belcher, play it and win on its activation.
So yes, I do believe if a deck is designed entirely around a single card for the win, than that card is generally considered a 1 card combo.
