The other commenter clearly implied 2 things:
1. sea mammals have salt glands
2. their bodies have solved the problem of removing waste salt from said glands
There's no way to interpret the commenter's statement otherwise without it being nonsensical. Sea mammals manage (what exactly) just fine?
If you can't see that, then I'm sorry, here's a simpler analogy to illustrate my point:
Me: "Pancakes can't be made without flour"
Commenter: "French chefs manage just fine"
A rational person would deduce that what Person 2 is implying is: "French chefs [are able to make pancakes without flour] just fine", not: "French chefs [are able to make pancakes] just fine"