Entanglement and quantum causation are subjects that are near and dear to me, but this article is entangled with both Berkson's paradox, which is simply that there exists a hidden bias in a statistical set due to selection, and the notion of a common cause or quantum source for more than one entangled quantum observer.
This mix of classical bias in statistics that confuses causality and the quantum superposition that binds a source to an observer and also confuses causality seems to obfuscate rather than clarify quantum causality.
In the referenced paper, a single photon exists in a superposition of polarizations in a Sagnac interferometer. Ring interferometers like Sagnac work with polarization as opposed to spatial decoherence, so that means even more complex physics to argue about.
The bonding between atoms in a molecule as well as the bonding between a source and an observer results from the exchanges of a single photons. Both time and space emerge from these actions and putting a Sagnac interferometer in between a source and an observer is just a really good way to complexify the simple quantum bond that actually is what holds all matter together.
Even quantum gravity is due to the exchange of a single biphoton pair between a source and observer, which are also an observer and source. Beamsplitters and dual slit measurements usually do not include the source in their quantum logic, only the photon and observer, and so these experiments are prone to the selective statistics of the Berkson paradox.
Once the quantum superposition includes both source and observer as well as photon exchange, space and time naturally emerge and these various complexifications become just crossword puzzles of science. Fun to solve, but hardly useful for understanding the simplicity of the quantum bonding that holds the universe together.