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.
> ...for suggesting sea mammals have a "gland" that filters salt from 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.
You are mixing up the words “infer” and “imply”. But anyhow, no the other commenter did not imply that sea mammals have extra salt glands that humans do not.