It's all based on coming up with two numbers that when multiplied say that this resource is downloaded 73 billion times a day. That's 20 times / day for every internet user (even those using it on a 2G connection once a year). Given a reasonable caching period of 1 month, that'd mean the average user visits 600 different sites a month. That seems like much more than one would expect.
Or to look at it another way, global IP traffic is apparently estimated at 72000 PB/month. That's 2400PB/day. This font alone would then be accounting for 0.3% of all internet traffic, or 1% of all non-video traffic. Again, that's a very high number.
But at least it doesn't appear to be a totally impossible number either, just an awfully implausible one.