She could have been invited but did not RSVP until the last minute or she could have been the plus one to her friend Kelly. Of course you can always find a detractor, but it does not seem like she was unwelcomed.
People seems to have been subtly shady towards the Sussexes lately. First revealing that Harry emailed them directly and now the need to reveal that Meghan's appearance was a "surprise" and the comment about her being escorted out by a side door.
If Meghan's appearance is a clapback, my guess is that it is a clapback to the recent
"Where’s Meghan?" article in The Telegraph. I checked and
The Telegraph article also preceded
this People article. I thought that
The Telegraph article was beneath that publication and them straying into tabloid territory, but if my speculation is correct, it seemed to have struck a nerve.