This is a common misconception. First-to-file does not mean that prior art doesn't matter. First-to-file means that, if I invent something today, you invent it tomorrow, you try to patent it the next day, and I try to patent it three days from now, nobody gets the patent. In first-to-invent, I would get the patent.

The 2013 law explicitly still gives weight to prior art.

You can invent something without it being prior art if you don't publish how to do it, file a patent, or use/sell it in commerce.

So if you are testing a new invention in secret and then the University of Wisconsin files a patent on the same thing, they get the patent. If someone else had already sold the products or described it publicly, nobody could get a patent on it (except for the person who sold it and then only for a year after they sold it).