Why not Past Perfect: "For a Muslim hacking collective ..., which previously disabled the ... service last month, it was “challenge accepted”

Question

From the INDEPENDENT news "Muslim activists hack Isis mailing list hours after terrorists claimed it was unhackable" (highlighting by myself):

After numerous cyber attacks and official takedowns targeting its Amaq “news agency”, Isis issued a message on Friday night claiming it had increased security.

“In response to recent events, we have imposed more stringent security measures on our systems,” said the email in Arabic. “We can now handle email attacks or any type of hack.”

For a Muslim hacking collective called Di5s3nSi0N, which previously disabled the automated email service last month, it was “challenge accepted”.

As "it was" is already past and the automated email service had been disabled previously, showing respect to all my English grammar books, a Past Perfect form of "disable" should be used in the sentence highlighted boldly above. Why it is not?

Answer 1

The phrase "which previously disabled the automated service last month" is a relative clause which acts like an adjective modifying "hacking collective". It's not a necessary part of the sentence, but instead adds relevant information.

This means that the sentence is not structured in a way that requires the past perfect, for example:

[The collective] [had disabled the server] before [accepting the challenge]

Instead the structure is fairly simple:

For [the collective], [the increased security] was [a challenge].

plus some additional information about the collective's past exploits.

Similar sentences with relative clauses:

The band, who played before huge crowds just the previous year, now found themselves without an audience.

The ring, which was set with all kinds of previous stones, was lost after the fire.

Answer 2

When describing one event following another in the past, we can show their relation by using the past perfect for the earlier event, or else we can use the past tense for both, relying on a conjunction (e.g. after, before, when) to show which event took place earlier. Roughly speaking, the Past Perfect is not mandatory if time relation is clear from the context. – Mv Log

The collective had disabled the server before accepting the challenge: You are not obliged to use past perfect because we know what comes first past perfect can be used to emphasize the completion of the action, the time relation is clear as said Mv Log – user5577