Can scarcity apply to software? Bitcoin can be forked many times and the code can be copied to create new coins. It might be that bitcoin is similar to fiat where its value is determined from the trust of the people who uses it.
Each BTC isn't really
just software anymore though, much like fiat isn't just paper. Even if there were identical forks, people won't consider them as Bitcoin. Trust certainly plays a big part (maybe biggest, even) in valuation, but scarcity does too.
I think the comparison is subjective and for that reason doesn't really makes sense.
I think arbitrary is the word you're looking for. It probably would have made much more sense if they divided the total land area of the world by 21 million to highlight that there's only ever going to be
this much, regardless of value. I really thought it was a statement about scarcity rather than their own valuation.