Calckey is a Misskey (https://misskey.io/) fork, hence the name. Misskey is an extremely popular fediverse microblogging service from Japan with a lot of functionalities that Mastodon doesn't have (quote toot, emoji react, ...), in fact if you follow any Japanese accounts on Mastodon there is a good chance they are using Misskey instead
Basically Calckey diverged enough from Misskey (and I suspect Kainoa didn't like the name) that they agreed to rebrand. Kind of a big deal tbh