a problem only counts as solved when it's actually gone.

And there are a surprising number of problems that disappear once you have clarity, i.e., they are no longer a problem, even if you haven't done anything yet. They become, at most, minor goals or subgoals, or cease to be cognifively relevant because the actual action needed -- if indeed there is any -- can be done on autopilot.

IOW, a huge number of "problems" are merely situations mistakenly labeled as problems, or where the entire substance of the problem is actually internal to the person experiencing a problem. For example, the "problem" of "I don't know where to go for lunch around here" ceases to be a problem once you've achieved "clarity".

Or to put it another way, "problems" tend to exist in the map more than the territory, and Adams' quote is commenting on how it's always surprising how many of one's problems reside in one's map, rather than the territory. (Because we are biased towards assuming our problems come from the territory; evolutionarily speaking, that's where they used to mostly come from.)