Claiming “you never forget hard problems you solved” underestimates forgetting

Influential people, tech giant often say that "if you forgot some difficult problem, and you do not how to explain it probably you have not done it" seems as underestimating power of time and forgetting. There is that saying "if you do not use it you lose it". How many times have you solved, something difficult only once, because you needed it only just that one time, then fast forward 10 years later, you have just a vague feeling, but you cannot remember the details?