The article says "The fuzzier atom rustles more easily and records the path of the photon. In tuning up an atom’s fuzziness, researchers can increase the probability that a photon will exhibit particle-like behavior".

I think we're just seeing decoherence in action here. If the photon interacts with the atom, it becomes entangled with the environment (the atom). Giving the atom a higher temperature results in it having a higher probability of it interacting with the photon, and decohering.

And I think the individual photon doesn't have a mixture of a certain % of wave or particle like nature. It's just that there is a certain probability that it will decohere (interact with the atom), so if you turn up the temperature of the atoms, you'll just see a greater % of the photons decohering when they interact with those atoms.

That's just my amateur understanding of the situation, so I'm happy to be corrected by someone who knows better. Also, I don't have access to the paper itself (https://journals.aps.org/prl/abstract/10.1103/zwhd-1k2t) as it's paywalled and not on scihub.

Quantum mechanics is fascinating!