This guide mixes two different cases, and does not explain them very well either.
Он мог услышать вещи для себя довольно неприятные (Лермонтов).
There is no comma here not because the modifier is tightly linked to a subject (as the guide suggests), but for two reasons which must play together:
The extended modifier (распространённое определение) as a whole is a comment (рема) and as such has to be put towards the end of the sentence, which is not how Russian concording modifiers (согласованные определения) normally behave.
The modifier itself (adjective or participle) is the last word within the clause, so that all other words, dependent on it, should precede it.
The reason 2 was what the guide was really about.
The main idea is that sentence, as it goes, keeps putting the words so that the topic does not have to be separated using intonation, hence no need for isolating it with a comma.
Let's have a closer look.
The author has put the sentence in this order because неприятные is the ultimate comment of the sentence, and he wanted to put it into the very end to give it the right intonation. To do that, he has to violate normal Russian word order. Normally, the sentence would read он мог услышать довольно неприятные для себя вещи, but this would make вещи the comment and неприятные the topic. Hence, the word order reversal.
Now, what would have happened if the comment would have been для себя? It would have gone into the end of the sentence, like this:
Он мог услышать вещи, довольно неприятные для себя.
Довольно неприятные для себя is a normal Russian word order. So as the sentence goes, we have a topic (вещи) followed by another topic (неприятные), and, hence, we have to intonate the main topic.
Same happens if we are to make довольно the comment. We can't just put it to the end of the sentence, but we can replace it with an adverbial clause:
Он мог услышать вещи, неприятные для себя в достаточной степени.
Same rules apply and we have to separate it with a comma.
Море у его ног лежало безмолвное и белое (Паустовский).
The second example uses a notional copula (знаменательная связка). It's a verb which plays both a grammatical role, connecting the subject (море) with a predicate (безмолвное и белое); and a semantic role, that is can be a predicate on itself (море лежало is a valid sentence). The adjectives are predicates here, not the modifiers, hence we don't need a comma.
Compare it to:
Море было белое и безмолвное.