It wasn't exactly a first string lineup fielded by Sri Lanka, probably a half strength lineup; I wouldn't give as much credit to Zimbabwe for almost winning a match as to Sri Lanka for fielding just enough strength to win every match.
That's how they can make the most of their talents. They've been quick to get these other guys into the game for dead rubbers and minor matches, so when it really counts, they've got experienced players to fall back on or call upon when they come of age.