Another is "supported by." As in 'Bitcoin is supported by the scarcity of energy available from natural resources.'
Back to the first definition (which is what most people use): what exactly and by whom is guaranteed to be exchanged for irrevocably spent electricity in case of Bitcoin?
The first definition only (strictly/literally) applies to currencies legislatively or constitutionally tied to a commodity, IE dollars on a gold standard.
The second definition subsumes the first, is more general, and more useful because it is more commonly used.
The second definition is not "made up." You are merely confusing your own lack of education with objective reality.
Since we're going by common usage, no one says that the US Dollar is backed by its anticounterfeiting technology, even though according to the same reasoning, that limits its supply and supports its price. This is not even close to a common use. Why would we say that for bitcoins? The only way we could is if we're going
against common usage.
Even ignoring that, speaking of guaranteeing scarcity, mining doesn't guarantee scarcity, since the 21 million limit is there regardless of the ability to rescind transactions, which is what mining actually tries to prevent.