Well, that summary is too much simplified. As a famous linguist 寺村秀夫 wrote "it is the most complicated thing how the tense or aspect of a verb or adjective appears in a modifying clause/phrase, and the rules are delicate (連体修飾をする動詞や形容詞におけるテンス、アスペクトの現われ方は、最も複雑で、きまりも微妙である)" in his book, it's actually far more complicated.
First, not only the relative tense (相対時制) is used in Japanese. The absolute tense (絶対時制) is also used. See the following examples.
1. 昨日開放感に溢れていた教室に、去年までは折紙がいた。
2. 昨日開放感に溢れていた教室に、今朝折紙が現れた。
3. 昨日開放感に溢れていた教室に、たった今折紙が現れた。
4. 昨日開放感に溢れていた教室に、明日折紙が現れるだろう。
Regardless of the tense of the main verb (1 and 2 are past, 3 is present(present perfect) and 4 is future) or the temporal order of the two events (1 is "the main verb --> modifying verb", the rest are opposite), the past form 溢れていた is used in the modifying clause. This is actually the absolute tense. The reference point in time is when it's said or written.
Secondly, there are cases where the reference point in time of the modifying clause is neither when the main verb occurs (主節時) nor when the sentence is said/written (発話時).
5. アメリカに留学する夢が去年やっと叶った。
The temporal order is not 夢が叶う --> 留学する, so the reference point is not 主節時, and it's not 発話時, either, since 留学する is a past event (the subject's dream "studying in the US" was fulfilled last year).
Actually, the reference point is when the subject had the dream here. 留学する is future of that time, so the present form(ル形) is used. Some linguists call this reference point 主名詞時基準, and this is also classified into 相対時制. Thus, the key of 相対時制 is that it's not 絶対時制, and the reference point of 相対時制 is not always 主節時.
Thirdly, there are cases where both the present form(ル形) and past form(タ形) can be used for the same meaning.
6. 明日の授業を取る人は、明後日は取らなくてもいいです。
7. 明日の授業を取った人は、明後日は取らなくてもいいです。
8. アフリカに赴任する社員は、皆、事前に予防接種を受けていた。
9. アフリカに赴任した社員は、皆、事前に予防接種を受けていた。
#6 and 9 are 絶対時制. The event in the modifying clause is future of 発話時 in #6, so the present form 取る is used, and it's past of 発話時 in #9, so the past form 赴任した is used.
On the other hand, #7 and 8 are 相対時制. 取った is used in #7 since the temporal order is "modifying verb --> main verb (明日取る --> 明後日取る)", and 赴任する is used in #8 since the order is "main verb --> modifying verb (予防接種を受ける --> 赴任する)".
This doesn't mean that both the present form and past form are always valid, of course. There are cases where only either of 絶対時制 or 相対時制 can be used.
As you can see so far, the past form in the modifying clause does not always show the temporal order of the two events. I only explained about usual actions in this post, but there are different rules for a state/an event that has a relatively long time period, and ~ている form often expresses a state, as you know. Furthermore, you have been asking about narrative part in novels, which also has special rules different from usual speech.
Finally, here's an excerpt from another linguist's book.
日本語には、各節が表す事態ごとに眺める視点の位置があり、それを表す事こそが、ル形、タ形の本来の役割なのである。すなわち、日本語の時制は、そもそも発話時や主節時といった特定の基準時を持つのではなく、事態との相対的な位置に視点を持ってくるものであり、ル形は事態の途中又は事態がこれから起きるという時点に、タ形は事態が終ったものとして見える時点に、認知主体の視点を導くマーカーの様なものである。実は日本語というのは、事態を見ている視点の絶対的位置に関してはどちらかといえば無頓着なのである。
She pointed out that Japanese language is relatively unconcerned about the absolute position of the reference point in time. The reference point can be different even in each clause in a single sentence, and ル形 or タ形 work as a marker to show the reference point.
Sorry for a long post, but I hope this is somewhat helpful for your understanding of Japanese tense system.