First, I dont want this to be true but it is.
Currency is a unit.
It is a unit in the system of measurement of value. Just like an inch is a unit in the system of measurement for length.
Units (and there have been thousands of different ones) all have 2 things in common.
1- They are not real. They are all made up, everyone of them is just an opinion. Inch, pound, meter, cat 5, g force,currency --- not real.
the are all just an opinion that we chose to share. We all share the opinion that an inch is so long, if we all shared the opinion that an inch was a foot then it would be.
2- All units are equal to a constant and are objectively definable.
Bitcoin is not a currency because it is not a unit. It has to be definable it has to be equal to something. An inch is not real but it is definable.
* note that the USD no longer fits the definition of currency either. It is not a UNIT
Did the dollar ever fit your definition? From your "no longer" statement, I assume you'd say that it did under the gold standard. But that's just a few applications of the transitive property away from the same issue... What's an ounce of gold? Ok, it's 8.53x10^22 gold atoms. Great, what's a gold atom...79 protons, 117 neutrons. What's a proton? Some up-quarks, down-quarks...etc. They're all mental approximations/constructs. We've just found them to be incredibly useful. Bitcoin is no different. The abstract construct of there being 2.1 quadrillion units, defined as a mining algorithm, is just fine. Quarks are definable by math as well. Physics, hence reality, essentially reduces to math.
But that's all semantics. However, if you want to make your argument, you have to pay attention to semantics, since that's what your argument is.
It sounds like your issue is with the lack of physicality of bitcoin. You'd be satisfied if we were able to define bitcoin in terms of something physical (nevermind my argument above that that also becomes abstract at its core). If that's the issue, fine. I say it's irrelevant.
Various things and systems have been used for exchange throughout human history. What does the job well in a given time period for a given culture has varied tremendously. Bitcoin dramatically reduces the friction of exchange in modern times, and that meets the fundamental purpose of money, and practical definitions of currency, perfectly.