...when compared to a handful of fiat currencies, maybe. It's not fair to gatekeep with top performers, in my opinion.

There are many national currencies that lack stability. That doesn't make them "not currencies". It just makes them volatile currencies.

In fact, that's precisely why Bitcoin has become so popular in those countries—particularly Latin America; Bitcoin may not be stable, but it's a more reliable store of value (for now) than the bolivar or Argentinian peso.

I wonder though, if this relative stability is more to do with the stake invested in it from more stable currencies like Dollar, Euro and Yen.

In my head I can't get away from the idea, that beneath all the value vested in bitcoin is the actual olde worlde currencies that ultimately people need to change it back to in order to buy things.

When currencies experience hyperinflation, people stop using them. Even the people required to use them by law (wherever there is hyperinflation, there are always capital controls). They can still nominally be called currencies, but they are not much of anything.