So the Dovakhiin encounters the spirit of Saint Jiub who tells him that he did indeed rid Morrowind of every last cliff racer, but if you've also had Mjoll the Lioness as a follower for a significant length of time, she'll tell you the cliff racers in Morrowind make for excellent sport. I'm guessing there's between 170 to 195 years between Saint Jiub's extermination of Morrowind's cliff racers and Mjoll's cliff racer hunts in Morrowind.

???

If we take Morrowind's cliff racer extermination as a matter of fact. Where did more of them come from?

Because Vvardenfell is not all of Morrowind.

He definitely chased the Vvardenfell breed away. That's not something you can attribute to rumor or mistake. But mainland Morrowind is a big place.

He exterminated all Cliff Racers in Vvardenfell - he did not exterminate all Cliff Racers within all of Morrowind.

Morrowind takes place specifically on Vvardenfell. That's what he drove out.

There's still a huge landmass with alot more creatures.