Since OP didn't provide context, I have to assume this is the game in question:
https://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2759110272/43124676?tab=overview
What is so strange about them winning that way?
Somehow 0/2 Sejuani got killed by 3/2 Kai'Sa, 4/0 Viktor killed 1/1 Nami while they put resistance likely as 2v5. 5 players killed 2 less fed or underfed players, what a magic.
Then 5/0 Viktor killed 0/3 Teemo, how uncalled for. 4/2 Kai'Sa killed 3/2 Vayne, the closest you can get to tension. Except it was 3v5 at that point. Finalized by 0/3 Sejuani killed by 0/0/5 Renekton. While Camille seemed afk.
Not surprising 5:14 match ended this way. Not surprising match where one team steamrolled over the other ended this way.
So saying "One successful team fight at 20 minutes was enough to enable them to end the game in a single push when only 1 turret in the lane was down beforehand." is very far from the truth. Entire successful game allowed them this.
BTW as you can see the enemy had 1 turret where they had 3 turrets before the push, 2 dragons and the mentoined baron.