Introduced in 1999, the 5.3-liter (325-cubic-inch) Vortec engine is based on General Motors' "LS" engine platform that was launched in the 1997 Chevrolet Corvette. This Generation III design (and later Gen IV LS models) varies considerably from the earlier Gen I and Gen II configurations first produced in 1955.
You say potato, I say potato. I wouldn't put any gen I, II, III or whatever in anything I own, but I did own a Silverado and was told it was an L variant. The LS term gets used very loosely and covers dozens of small block itinerations, so maybe the guy selling the Chevelle took some liberties, but I think everyone but you understood where he was coming from.
Another thing, I have stated this here many times over the years, if you don't have the balls to say who you are and where you hail from, maybe you should be posting on the anonymous site.