In this case, "les escargots" means snails in general. There is a difference, I think, between manger and goûter. If you say "il a mangé les escargots", this must mean some specific snails, because otherwise it would imply he'd eaten all the snails in the world. But this doesn't apply to goûter, because you only need to eat a part of something in order to taste it.

Il a goûté des escargots. -- He tasted (some) snails.
Il a goûté les escargots. -- He tasted snails. (meaning snails in general)