The point is to parse the input into a structure which always upholds the predicates you care about so you don't end up continuously defensively programming in ifs and asserts.
Not at all, the article is about pushing complexity to the "edges" of your code so that the gooey center doesn't have to faff around with (re-)checking the same invariants over and over... but its examples are also in Haskell, in which it would be weird to do this without the type system.
In python or java or whatever you'd just parse your received_api_request_could_be_sketchy or fetched_db_records_but_are_they_really into a BlessedApiRequest or DefinitelyForRealDBRecords in their constructors or builder methods or whatever, disallow any other ways of creating those types, and then exclusively using those types.
edit: wait, actually no we agree, I must have glossed over your second sentence, sorry