[quuxplusone]

For "only two hard problems," read "two candidates for among the hardest problems (but we feel strongly that these are indeed good candidates)," or something along those lines, more or less.

It's also possible that these used to be the only two hard problems at the time the aphorism was first recorded, but the underlying state of the world has changed since then and the aphorism, as recorded, is no longer current.