The monetary value of gold emerged organically because it turned out to be the best available option for transactions between untrusted parties. After the emergence of modern states, this use case was restricted to cross-border payments since strong legal protections and centralized banking systems made fiat currencies trustworthy enough for domestic commerce.

But anyways, if one looks at why gold emerged as the preferred money for its time and place, “finiteness” aka non-zero marginal cost of production is certainly a relevant factor.

It does not follow from the post- gold-standard instability of gold that the prior stability was due to market manipulation; the nature of the asset changed fundamentally. You might say the gold was "backed by" the financial system, whereas now it's a speculative asset no better than BTC.

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.