Do you have a source for that? It sounds like something a climate denier would claim that the environmental interests are saying.
The North Polar Ice Cap could melt completely, and have hardly any effect on sea level. It's already floating in the Arctic Ocean.
The South Polar Ice, which sits on land, is of known extent, and thickness. The water amount (mass and volume) can be calculated easily enough; it's just arithmetic. It's just as easy to calculate the effect of dumping that volume of water into the world's oceans. High school math, and a lot of attention to detail.
I haven't done the math; I don't know if 600 feet is reasonable. But even 60 feet is enough to wipe out every coastal urban area on the planet, and put an end to our current civilization.