When you need to pee, you’re less likely to believe in free will.
Ridiculous as that may be, it fits into a consistent pattern. If you’re tired or horny, you (on average) believe less in free will. Likewise if you’re epileptic or suffer panic attacks.
The researchers are keeping their evidence secret, so let’s generalize a bit. The more you are at the mercy of your body, the less likely you are to imagine yourself as choosing your own destiny.
I’ve no idea how free will was defined (again, because paywall). But it fits perfectly into the wider story of how the belief in autonomy and free choice correlate with privilege. So you have billionaires convinced they have pulled themselves up by their bootstraps, while thanks to learned helplessness, if you’ve spent a lifetime being shat on, you probably expect it to keep happening whatever you do.