Could someone explain the semantic difference between "lake" and "sea" here? Why is this called a "lake" (or even "megalake") even though it was far larger tha

Could someone explain the semantic difference between "lake" and "sea" here? Why is this called a "lake" (or even "megalake") even though it was far larger than many of today's seas? (or indeed some of today's seas are fragments of the ancient lake) Edit: I guess it has more to do with salinity and whether or not the body of water is connected to the ocean?