Yeah, look for this part in the the article:

Lexicon validation works the same way. The com.tumblr in com.tumblr.post signals who designed the lexicon, but the records themselves could have been created by any app at all. This is why apps always treat records as untrusted input, similar to POST request bodies. When you generate type definitions from a lexicon, you also get a function that will do the validation for you. If some record passes the check, great—you get a typed object. If not, fine, ignore that record.

So, validate on read, just like files.