Don't confuse working at a topic with mindless repitition.
I remember dd endlessly doing column method (by choice, at home) and constantly writing sheets of sums for her, before she started making her own. Yes that repitition helped her, and as she was the one asking to do it, it clearly didn't bore her. Because at that time, it was sufficiently challenging it didn't bore her. If she'd first met it at the age it's taught at primary, and been asked to do that level of repitition, it would have been mindless.
Exactly like the repitition of a preschool book is stimulating and enjoyable when you're little. But mindless repitition if you got older, able readers to read that same 2 minute book over and over again.
It's so mentally deadening.
I also think we lose a lot of the most able before anyone can judge whether they have gaps in their knowledge in their gifted area.