But do you not think that he would have been quite an important enough guest to warrant being named?
"Named" by who?
It's not like Harry and Meghan sent out a press release saying "here's everyone who's attending the wedding."
Most of the names on the Wikipedia page are the result of the press scouring the videos of the footage and identifying all the people they recognise by sight. Watch the footage - the reporters didn't know ahead of time who would be attending (except for the obvious guests) and were trying to ID the guests they saw onscreen as those guests arrived.
Obviously British reporters aren't going to recognise Meghan's uncle by sight, which means the only way her uncle would be "named" is if either he or Meghan decide to issue a press release, which would be a very weird thing to do.
Her uncle has never spoken to the press and obviously wants privacy.