Ok, it seems like Kondo did have help, as Naoto Kubo and Asuka Hayazaki co-composed with him, per the
game's credits. So this means either Kondo forgot about their roles, or they both joined the project after Kondo claimed he was the sole composer in December (it's been 10 months since then, which is most likely).
Slightly off topic: Kubo was credited in Captain Toad: Treasure Tracker, but no roles were given for any of the personnel. It's highly likely he did help Yokota with composing, however, now that we know for sure Kubo is a composer and not a sound designer who did a few tracks for Majora's Mask 3D.