I don't think they pulled the number out of their arse. Yes, the maths behind the figure required a lot of assumptions and guesswork, but their assumptions seemed fairly logical to me.
Are you saying that you think bitcoin will become an international currency? I just don't think this will happen.
I'd also say, that they did pull it out of the ass, but wrapped it up in nicely looking reasoning. They made that valuation when BTC price stopped at around $1200, so their prediction of $1300/BTC looked pretty sensible back then. I can guarantee that if they did the same kind of valuation, say 6 months earlier, the number would be significantly different.
The simple truth is, to make such assessment, they need to do a lot of guessing, like this one:
were to become a dominant medium of exchange for e-commerce that accounts
for, let’s say, 10% of all the payments for B2C transactions?
Why not 2%, or 0.3%, or 20%, or 75.14%? They didn't quite explain that, did they? Or did I missed something.
Also, you can argue with the listed Advantages/Disadvantages, these have been widely discussed on this forum.