The point is that there was no underlying law of nature attaching gold to the financial system. The only thing "backed by" meant was, the government promised to trade gold and regulate the gold market as required to maintain their declared peg. Maybe it's a bit of a "hot take" to call that manipulation, but I don't think it's untrue or unfair - in some countries (although not the UK) governments went as far as simply prohibiting private speculation in gold.