It's absurd to suggest that papal responsibilities include debunking every myth out there. JP II specifically said he was talking about the Apocalypse.
It's not absurd at all. That's the Pope's job.
You know that this "myth" is almost universally believed. It is not a traditionalist invention. It's been going for at least 7 decades according to Fr. Cekada. And Fr. Shannon Collins on EWTN just recently relayed the "myth" again as fact on his show,"Does the Church still teach that?" Also the Adoremus society believes JPII was referring to the vision of Leo.
And, I'll have to double check but I believe Fr. Gabrielle Amorth, the chief Exorcist of Rome under JPII mentioned Leo's vision in one of his books.
The question is why would the Pope not take an opportunity to quash a myth surrounding his predecessor and instead make a comment that can be seen as a validation of that myth?