Compared to before the Pleistocene? Humans decreased biodiversity.
But if rewilding is meant in the pre-Columbian sense, then the hypothetical would be to compare the ecology in the year 1400 had humans in the Americans died out in, say, the year 900.
We simply don't know the answer to that, that I know of in my limited understanding.
We do know that human fire practices change the landscape. https://en.wikipedia.org/wiki/Greater_Yellowstone_Ecosystem#... mentions the decline of quaking aspen, and how one of the factors may be fire-return intervals. Humans have long used fire to clear out underbrush, but I really know so little of archaeology or related fields.
So in that case? Humans may have help prevent biodiversity reduction.
I can point towards Australia as an example of a longer-term ecosystem adaptation for human fire practices. The general belief is that "Burn-offs or small, cool fires were thought to be used by Aboriginal people both for hunting and for the prevention of larger fires" and "most Australian plants wouldn't be distributed around the country the way that they are now if people hadn't burned-off the land through history." - http://www.australiangeographic.com.au/news/2010/12/aborigin...
That's not saying there was more or less biodiversity, only that there are different ecosystem patterns with and without human. But I don't see how it's possible to compare biodiversity when there's been 10,000+ or 50,000+ years of ecosystem adaption.
Is there less biodiversity in North and South America because of the Great American Interchange? Should pre-Panamanian land bridge be our goal once we've restored the Pleistocene era ecology? There had to be a goal besides "improved biodiversity." My goal, btw, is to point out that humans are part of the ecology.