Sentence Correction

I am not clear on the explanation to the following sentence in MGMAT SC guide.

The band U2 WAS just one of many new groups on the rock music scene in the early 1980's, but less than ten years later, U2 HAD fully ECLIPSED its early rivals in the pantheon of popular music.

From what I understand about past perfect tense, the earlier action should be the one that is expressed with HAD + past participle. However, in this sentence, it looks like the earlier action uses a simple past (WAS). Is this an exception ?

nox104 wrote:I am not clear on the explanation to the following sentence in MGMAT SC guide.

The band U2 WAS just one of many new groups on the rock music scene in the early 1980's, but less than ten years later, U2 HAD fully ECLIPSED its early rivals in the pantheon of popular music.

From what I understand about past perfect tense, the earlier action should be the one that is expressed with HAD + past participle. However, in this sentence, it looks like the earlier action uses a simple past (WAS). Is this an exception ?

Well,over here the use of "had been" doesn't look appropriate.
"The band U2 had been just one of many new groups"
Also,it is followed up by "just".
So "was" becomes the choice over here.

It would be more clear if you understand what the author is trying to convey.

Well, I would like to hear an explanation for how this sentence is correct grammatically w.r.t the use of past perfect tense.