in a corner of the room sounds like a ludicrous (and
thereby humorous) proposition.
It isn't much more ludicrous (or humorous) than the idea that two randomly generated private keys will result in a collision.
It was not intended as a joke. It was intended as a conceptual analogy (since people seem to have a difficult time conceptualizing very large numbers).
Yes, it is a good analogy, although the air molecules would possibly
be orders of magnitude less likely. We'd have to ask a physicist.
I guess there are funnier comparisons... like the chances
you run into Bill Murray at starbucks and he just decides
to give you 100 BTC? Is that funny?
What about "as likely as getting attacked in your bathtub tomorrow by wild
monkeys wearing orange hats?"
Speaking of monkeys and probability, the probability that a monkey
would randomly type Hamlet by William Shakespeare on a keyboard
is about 1 in 10^360733.