I don't think the author of the article has chosen a good example. It is possible to re-write the given sentence with a parenthetical phrase ", written by Hemmingway,"
I think the article is confusing a separate fact, relative pronouns that are the subject of their clauses can't be omitted.
A relative pronoun can be the subject or the object in the relative clause.
If the pronoun "that" is the object of the relative cause, the pronoun can be omitted:
A book that I read = A book I read.
But if the pronoun is the subject, then it can't be omitted.
A person that reads a book. (not the same as "A person reads a book")
The difference is that in the first sentence, "that=book" is object of the verb "read", in the second sentence "that=person" is the subject of the verb.
It may be that the article is saying that you can't reduce the relative clause to a participle:
I gave a written by Hemmingway book to Mary last week.
In that, the article is generally correct. That is not a good sentence.