You are right, he doesn't need that much.

As for timelines, I think with enough motivation an entire GCSE can be devoured in a few months but the author is being realistic about the time: as an adult autodidact, you'd probably be doing it in addition to work/family/all the rest, so it can take longer.

Your also forgetting since children study many subjects, they probably only study fewer than 6 hours of maths a week. And half of that is probably low quality study.