They don’t really debunk it as much as they analyze it. the parts that I find the hardest to debunk it the second secret which says that (Russia will spread her errors throughout the word causing many wars
But at the end Russia will be converted and the world would enter a period of peace) Now Russia did spread communism throughout the world and the soviet union collapsed.
Why is it so important to you to “debunk” it? Personally I’d settle for understanding it.
Read the prophecy more carefully. It says, “If my requests are heeded, Russia will be converted, and there will be peace; if not, she will spread her errors throughout the world, causing wars and persecutions of the Church.”
Seems pretty straightforward to me. It’s common practice to single out the Big Enemy of whatever time you live in and say, “Yep, one day they’ll see the error of their ways and they’ll bow down in submission…” If I recall correctly, the Old Testament prophets used to say the same thing about Egypt and Babylon.