Is there anything at all to this? Spotted online, no idea how I'd even begin to go about fact-checking any of it.
Yeah, probably? But it isn't a big deal, really. I probably am, you probably are, the Queen of England probably is... I can't put an exact number to it, but most people alive probably are... As discussed in this old answer while there is something marginally notable in being lucky enough to have surviving, verifiable documentation of ones line of descent back beyond the most recent few generations, the simple fact is that when we're talking about people who lived more than 1000 years ago, as long as they had descendents, we're talking about incredibly wide swatches of the earth's population claiming descent, rather than a few notables.