You are correct, that is application sandboxing. Please understand that this problem isn’t an issue with darktable itself, bit rather a feature of ubuntu’s snap packaging format. Sandboxing can be used for any snap application to restrict the applications access to parts of the system. While it may not make sense for darktable, that was the decision made by the snap packager. For some applications, it is a great feature, for instance with the Steam gaming app, which phones home a ton of analytics, sandboxing limits what it can see.