Given the fact that this is a combination of well-known techniques, trivially offers performance benefits, and makes the transformation part of the code testable in isolation from the process or recursion, it's a smart first pass. I think most experienced OO engineers could sit down and write this in one pass.

Calling it overengineered is pretty ridiculous IMO. "Over-engineered" doesn't mean, "Because software training is often inadequate someone might encounter this for the first time in industry rather than in school."