Emulator's would warrant a ROM, therefore they don't release because of such things because it (ROM's) could be illegally downloaded. It's fine if you own the game, but a majority will take advantage of it.
Also, the only advantage of having an emulator for the console systems is backwards compatibility. They did it for the PS3 for PS2 games. But not for PC.
Most PCs are more powerful than a 360; however the Playstation 2 is very old and slow comparatively (it runs at 300mhz), and it still needs a powerful PC (3.5ghz to 4.5ghz) to emulate it (as does the N64 and WII etc). Recreating hardware functions in software and interpreting is what sucks up so much power.
Not even the fastest 12-core processor would be powerful enough to recreate 360 hardware in software. I wouldn't expect an emu for it for another decade.
MS may have been talking about a development kit - software studios develop 360 games using special hardware that isn't available outside of their dev kits.