That's just how market caps work in general. The same argument has been made for startup valuations. It's not a problem with the metric at all and doesn't hold for the case being discussed as actual market based transactions in enough volume are happening. It's not just a made up number. Here's an argument that Facebook was not worth 33B$ in 2010 because only 3% of its value had ever been transacted:

https://signalvnoise.com/posts/2585-facebook-is-not-worth-33...

It's worth almost 17 times more than that now. Market cap is not a perfect metric because it doesn't capture the depth of the market willing to buy but coins don't make that any different.