My reading from the License section[1] of the Keepasium README and this Stack Exchange post[2] is that the author of KeePassium wishes to license KeePassium under GPLv3. Accepting applications licensed under GPLv3 would require that Apple provide certain forms of source code alongside App Store downloads which they are unwilling to do. As such the App Store terms of service has terminology stating that you give Apple the right to not do that, which is something that only the copyright holder(s) of a work can do. The simplest way to have clarity over who holds the copyright is to have a single author. So long as the KeePassium author is willing to assign Apple the permission implicit in submitting to the App Store, that’s fine. It just means that all other uses of KeePassium must follow the GPLv3 license.
I am not a lawyer, nor really even well-versed in IP law, and you should not take this as legal advice.
[1] https://github.com/keepassium/KeePassium?tab=readme-ov-file#...
[2] https://opensource.stackexchange.com/questions/9500/is-apple...