https://platypus.asn.au/platypus-myths/

The fact is that the platypus’s digestive tract does include a small expanded pouch-like section where one would normally expect a stomach to be located. The platypus’s stomach doesn’t secrete digestive acids or enzymes (Harrop and Hume 1980; Ordoñez et al. 2008), but does produce a mucus-rich fluid to assist nutrient absorption in the intestines (Krause 1971). Following on from the discussion of grinding pads above, it would seem that a platypus masticates food so thoroughly in its mouth that little additional processing is required before food reaches the intestines. Also, because a platypus consumes numerous small prey items over a period of many hours, its stomach doesn’t need to have a large holding capacity to accommodate infrequent large meals.

Sooo, "gullet"?

A pseudo-stomach? IDK...

I think since it's using bacteria and not acid, it's not a "stomach", just performs the same type of function.