It's saying that Christianity has a prominent martyrdom narrative, Christianity is hegemonic in the west, so therefore western leftists view martyrdom positively. It's not saying that all martyrdom narratives are Christian.
It's like saying that English uses pronouns, English is hegemonic in the U.S., so therefore U.S. leftists use pronouns. That doesn't imply the author thinks pronouns are an exclusive feature of the English language.
If A, then B.
Observation: B. Therefore, A.
That's what the logic of the text is saying.
It's saying that Christianity has a prominent martyrdom narrative, Christianity is hegemonic in the west, so therefore western leftists view martyrdom positively. It's not saying that all martyrdom narratives are Christian.
It's like saying that English uses pronouns, English is hegemonic in the U.S., so therefore U.S. leftists use pronouns. That doesn't imply the author thinks pronouns are an exclusive feature of the English language.
A vast majority of US leftists use English.
Does a vast majority of western leftists adhere to Christianity? No. So your metaphor really breaks down there.