Here's the deal: Anyone can claim to have power. Whether legal, spiritual, physical or Jedi-power. Heck, Jacob here has claimed all kinds of things.
Sadly, this is not enough to make it legal. You still need to prove to a court or other higher power WHICH HAS THE LEGAL POWER TO RECOGNIZE YOUR CLAIM that you really are who you claim to be or that your organization has that power.
Like Jacob, Boniface made claims but had nothing to back them with. Some may have cared that he excommunicated anyone and everyone who annoyed him but it apparently had little effect on his claims and France (with which he had his main beef) never acknowledged him and instead declared him to be heretical. Which to me seems to have a good case as he tried to do stuff that is way out of line for the pope.
In fact, if we go back to the whole "others have to recognize and approve the power" we can discuss how countries are legally formed. Note that other countries have to accept your country in order for it to be able to do anything with them. If I declare that I am "the sovereign nation of silly but impressive hats" that doesn't really work unless other countries start to say "yeah, that's fine by us. Here's an embassy and a trade deal you silly little fellow." In fact, it may be that I get a visit from the Finnish police or mental health workers who explain to me why I am being a silly boy (and I may be asked to come see a doctor or forced to do so if this is deemed necessary for my well being or for the well being of others.)
And again, countries can die and disappear. People can die and disappear without a heir in which case the country takes whatever property they have and takes care of the burial. Similarly when a nation dies it is not necessarily any of the organizations it has that takes over the region. When USSSR collapsed it wasn't any of their organizations that took over, instead new countries were formed. Note that though many of the organizations still exist (sometimes under new name) none of them controls the regions of former USSSR and instead they are part of the countries within which they reside (and even then only if that country decided to take that organization rather than disband it.)