A good point made about where the real losses are, but the Guardian writer in saying, 'Don't lie to yourself', should in turn not tell a lie.
The blanket statement that 'file sharing is a form of theft' is untrue and misleading. There are many, and possibly a majority, of exceptions that demonstrate the assertion is a lie. The principle of file sharing is very legal.
File sharing is not, and never has been, illegal. At the very most it's only illegal if the owner of the copyright of the file has expressly refused permission for the sharing of the file. Even then it can be argued that the common law permitted the individual, once having purchased an item, to do as he pleased with it, so long as there was no profitable compensation in return at the expense of the person owning the rights to the original material. That the law says otherwise now is a result of the robber barons moving the goal posts of the law, not because of a moral certitude.
Making something illegal doesn't make it morally wrong.