So you're saying the software designer(s) behind Timeshift were too stupid to design it so it couldn't back up the system disk to the system disk? Because it doesn't seem to me like it's the sort of thing which requires that much intelligence or foresight.
As an example, take a look at USB Image Writer. If you don't have a flash drive attached, it won't provide you with a list of targets to write the ISO data to. In theory, one could choose to write ISO data to one's system drive, but that program's author(s) weren't stupid enough to just let you do that by not bothering to restrict candidate storage mediums.