It would be better to ask them that. But I've seen this in several trad and neo-Cat places. No one ever provides where the source document is, though.
You don't have to be a lit crit to realize that no fifth-century author would ever say "After the year 1900, toward the middle of the 20th century." They just wouldn't.
So even if there is some sort of sermon castigating sinners, or even Nilus' actual prophecy, you know the text has been tampered with, at the very least.
But to me the whole thing sounds contrived. When I first saw it, I tried to search out a source, any shred of a textual source, behind it, and came up empty-handed. Everyone just repeated the whole thing verbatim. Some said it was approved by Pope So-and-so, but that thread led nowhere.
It also started circulating fairly recently (late-80s or mid-90s, I think), and does not appear in, say, Catholic end-time books from the '50s.
I'm even a bit suspicious of the use of the name "Nilus." It sounds to me like whoever invented it was making an "inside joke" about the Protocols of the Elders of Sion, published by Sergei Nilus in 1905.