Sea mammals get most of their water from the food they eat (fish, etc.), and don’t drink seawater.
They do supplement that with water extracted from their food. But unlike humans they are better at expelling salt. “In the seal and sea lion species, for which measurements exist, the animals' urine contains up to two and a half times more salt than seawater does and seven or eight times more salt than their blood.” Human urine on the other hand contains less salt than sea water resulting in dehydration when drinking sea water.
No-one made any such suggestion. One person lamented that such a gland didn't exist; they did not assert its existence. The other person didn't mention glands at all.
That infers sea mammals have working salt glands.
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"