It means in a browser with a rendering engine made for that one platform, and not made more generically for cross-platform support. That is the most charitable

It means in a browser with a rendering engine made for that one platform, and not made more generically for cross-platform support. That is the most charitable reading, and how even that reading is supposed to be meaningful is beyond me. Basically, since MS can't demonstrate in any meaningful way that their not-cross-platform engine does anything better, they're offering up "it runs better because it's made for Windows" as a truism.