I've no idea about the truth or otherwise of that, but the articles I've read invariably seem to have been written by people from London in their mid-20s. And although they mention the song the claim is that "Man U" is offensive in itself (I guess calling them Manure is self evidently offensive...
I have read it before on the internet, and dismissed it as total nonsense.
You can get a measure of how ridiculous this is from how the actual claim isn't that "Manure" is offensive, it's calling them "Man U" that we're all meant to avoid.