Flood the house from what? In theory, if you simply cut the siding to size and fit a vent into place, rain water can seep through the siding roll down the paper and get to the wood instead of continuing all the way down to the weep screed at the bottom.

So technically you should cut back the siding enough to flash the vent properly then redo the siding. But you're not going to do that, and in all likelyhood you will never see a problem.

Was that what your friend meant?