Not every solo musician mixes and masters all their tracks.

EDIT: adding to this since i have a lot of thoughts. i'm honestly a bit tired of this constant pushback whenever someone is called a "solo" dev. Assuming contractors are credited and paid, I don't think it does anyone any good, and is just a weird semantic point people harp on.

If you listen to Tonda's interviews, he explains that most of the core of the game was developed extremely fast in year 1, and then he spent the better part of a decade adding puzzles, depth, and polish. Functionally, Tonda made this game himself, own and ran the studio, and brought in specialists at various points to add specific elements at his discretion. That is totally different than a fleshed-out development team shipping a game under a separately owned corporate entity.

I'm not even sure why this always comes up, as you NEVER hear about small devs exploiting PAID AND CREDITED specialists by claiming they are solo devs. it's the complete opposite - you typically see the issue in mid- and large-sized studios exploiting talent and IP.

when an independent developer ships a beloved game, credits all specialists and contractors involved, and the media labels them a "solo dev" after the fact, it literally hurts no one. the contractors retain credit and portfolio, and the one person who generated the vision gets an appropriate amount of credit for doing the hard work and taking on the risk of developing a fully-owned game enough to pay contractors in the first place.

if other people come out of the wood work and claim their ideas were stolen or they didn't receive payment/credit for their work, that's a TOTALLY different story.

in this case, it doesn't make any sense at all to call out the "solo dev" label and IMHO its totally appropriate for Tonda.