> I have a hard time believing that a pure SPA approach would be simpler to implement.

IF you used a pure SPA approach with client-side validation for each step, and server-side validation only done at the last step, I believe it would be simpler.

However, let's say you introduce anything slightly more complicated. Like say you do server-side validation with each step, now you have to somehow persist that "validated data". In that case, the implementation in the article is indeed simpler (or at least not as complicated as a traditional SPA).