What does “by a landslide” mean in English?

This documentary says dogs were “the first domesticated species by a landslide of thousands of years.” A landslide is rocks and earth sliding down a hill, unstoppable once it starts. By a landslide borrows that image for any huge, one-sided win: an election won by a landslide has almost no real contest. Here, “thousands of years” is the landslide: no other animal comes close.

Normally the idiom describes an overwhelming win, especially in an election: “she won the vote by a landslide.” This documentary uses it playfully for time instead of votes: dogs weren’t just the first domesticated animal, they were first by thousands of years. The image does the work: something so big it buries everything else.