Whilst the application may not read/write arbitrary files, the removable-media permission is not able to be scoped to just a single file so either way it ends up with the full permission even if it doesn’t necessarily use it.
As per the Process for aliases, auto-connections and tracks, auto-connect for removable-media is currently only defined as appropriate for major browsers / email clients and applications that handle media files. Since KDiskMark does not fit within this category, I do not think it is appropriate for auto-connect with the current guidelines.
Since the snap has mount-observe auto-connected though, it sounds like you are already able to prompt / notify the user when they may need to connect the removable-media plug and so this seems like it should be able to provide a reasonable end-user experience without auto-connect.
As such, -1 from me for auto-connect of removable-media for kdiskmark.