It's almost certainly a result Penguin picking a time that will be best for their balance sheet. There are industrial concerns (still gotta print, produce and execute a marketing plan, book speaking engagements and promotions, etc.) Penguin's got the juice it needs to rush all of that, but wouldn't if they didn't think it was worthwhile. They're likely thinking that Peace Talks is going to be a bestseller regardless of its street date, and they're probably right.
But they also don't want to cannibalize their own sales on other books; most readers don't buy more than one book at a time. So they want to find a good release window to prevent Dresden from negatively impacting the opening week (or weeks) of other books. Also have to look at competing books in the urban fantasy or adjacent genres; they want to make sure Peace Talks will win the week it's released.
The final reason they may be delaying is they may want to prop up an otherwise lackluster quarter. Releasing in January or February can be a good way to kickstart their Q1 (and 2020) revenue sheet. January usually has pretty good book sales, but it's mostly in nonfiction stuff (which dominates the sales charts as it is). So stuff like Jordan Peterson's Rules for Life or other self-help books will probably dominate book sales since everyone is still convinced that they're going to keep their New Year's Resolutions. But it's not a great month for fiction, after everyone's blown through their gift cards (and people are still recovering from Holiday shopping spending). February is usually a pretty weak month for books (and for all of retail except florists, Hallmark, and jewelers), and it's a couple shopping days shorter (only one or two since it's a Leap Year), so a major blockbuster release like Peace Talks will make them look good.
The good news for me is that my friend and I are going to reschedule our bet. There was a stipulation that if there is no announcement by Black Friday, we're going to have the opportunity to alter our chosen dates. So I might not be out five hundred bucks after all (my bet was on last Tuesday).