does deck sealers really prevent the deck from rotting the wood?

yes but you must reapply every year or so

They prevent the wood from getting soaking wet. But, if the wood is quite cracked, the effectiveness is much lessened, since the sealers don't get all the way into the crack.

Water is the critical component of rot, so, yes, sealers prevent rot to a large degree. If you are talking pressure-treated lumber, there's really no need to bother. If you are talking fancier wood, say Ipe, Mahogany, or Redwood, I would skip the typical Thompson's and go with Australian Timber Oil. Awesome stuff.