Their website describes this as open source but their linked repo is under CC BY-NC-SA 4.0 [1] so not commonly regarded as open source, but instead source available. They have been made aware of this [2]. Additionally, I think it may only be the front-end parts of their apps that are source available, I'm not sure the server-side parts of their app have sources published.
[1] https://creativecommons.org/licenses/by-nc-sa/4.0/ [2] https://github.com/skiff-org/skiff-apps/issues/94