Youtube is hardly the most reliable source, but it's entirely possible that it's true.
Some people have indicated that Mac cards (the newer ones, mind you) are compatible with PC's, just not vice-versa, because standard PC cards are missing firmware that the Mac needs to boot but everything Windows needs to boot is present on a Mac card. (See the "What is flashing" section in this post.
If you already have the card, it's worth a shot. However, like jsc said, unless you got it for a lot less than the price listed on that site you gave, you can get a brand new 5770 for a lot less.