Something I haven't seen anyone else mention is that this is problem 15. The author probably did the previous 14 problems too. IIRC the problems aren't strictly cumulative but you do generally end up warmed up and with hints about how to approach problem N efficiently from problems [1, N-1].
Skills do decay, I can't deny that. But even when I was still in school going back and doing an end-of-year problem for a class I took two years ago would have been harder than it was for me at the time... but it would have been easier than the first time if I warmed up properly with a bit of review and practice first, and I mean, not just three minutes glancing over things but taking some serious time for it.
The sequence numbers of problems on Project Euler only represent the order of publication. They aren't deliberately connected except for a few problems that have one or two follow-ups.
Most of the first 100 problems can be solved without any understanding of the problem, should you so desire.