The headline claims that a lake estimated to be 34 million years old was the largest ever on a planet billions of years old. Seems a little presumptuous.
It's the largest we have any evidence of ever existing, and it's also stupendously large- where and how could a larger lake have existed?
Just because we don't know everything doesn't mean we can't be pretty certain of anything.
It exists somewhere in the gap of your logic. The undeying complaint is against the superlativ, and not against "largest" but "in Earth's history", which equates to everrr.
It's clearly click-bait.