The deflected exhaust gasses do not contribute appreciably to slowing the aircraft. Some aircraft generate adequate vectored thrust in this manner to perform a push back, but even in these cases, it's not the thrust vector from the buckets, blockers, or cascades that's producing the retarding effect you feel with reverse thrust. It's the intake drag.
When you remove the forward thrust, your'e not doing so for the purposes of directing it opposite the direction of motion. You're removing it so that all you're left with is intake drag. The effect of exhaust gasses vectored opposite the direction of motion is minimal.