I mean, I think it’s exactly that, no need to look further.
I fully agree, leave them alone, especially if she’s absent due to health issues.
But people treat the royals like public property, and so they notice when photos of someone haven’t been released for months. So then she or the palace responded by publishing this picture, clearly to say “look, here she is, she’s totally fine and healthy.” (A dated newspaper casually in the background would have been a little too obvious, though I wouldn’t have put it past them.)
So then it’s a bit rich when this photo, clearly submitted as proof that she’s ok, has been faked.
The public shouldn’t be demanding photos of Kate, they should just let her be. But I think the juxtaposition of this photo’s intent and its effect is mildly noteworthy.