I agree, it makes sense that you’d have to drill the whole thing out to a larger size and that is a “retap” … except … the last (and only…) time I had a stripped yoyo fixed, it was a bit different than this… he said that the helicoil inserts new stainless steel threads in the same exact size hole! He even re-used the same axle size!
You still need someone that has a helicoil kit and an accurate drill press to get it right – any misalignment of the axle is obviously catastrophic!