I don't know where you got that idea, but we're not even the most populous species of vertebrates on the planet (that might be chickens).
If you include arthropods, ants make it not even remotely close.
"Land vertebrates", #1 is chickens, #2 is humans. But colloquially "animal" does not include ants or fish.
In terms of biomass humans are much larger than ants.
Humans and our cattle are something like 95% of all land mammals, maybe they're thinking of that.