Absolutely do not use flex seal to water proof your basement. I hope that guy was not being serious. There are only two ways to keep/get the water out. A correctly installed French drain that is backfilled correctly is the best. Obviously if you don't already have that then a quality sump is your only route unless you want to fork over a heap of cash to go digging around your whole house.
Gutters with downspouts draining well away will help, everything else is a bandaid. Good luck.