The word guarantee is irrelevant in the game of cricket because anything can happen in the game of cricket. You said Mumbai Indians win today will make the playoffs absolutely certain for them . you are guaranteeing them to be certain but you may have failed to match some equation here. 

Firstly, if Mumbai Indians win today's match, points will be 16 and RCB's number 5 in the points table will be 12. If RCB wins two consecutive matches, then Mumbai Indians and RCB's points will be equal, but considering the NRR, RCB will be ahead to , so RCB will have a chance to playoffs instead of Mumbai Indians.
Just winning today's match will not ensure them playoffs they have to win last 2 matches and if for some reason they fail to win 2 matches then they have to look at RCB match because if RCB loses one of the two matches then Mumbai Indians can qualify for the playoffs.