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.

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.