Quantum experiment shows events may have no fixed order

article writes:

"In this setup, a single photon is placed in a superposition of two paths using a beam splitter. Along one path, the photon experiences event A before event B; along the other, it experiences B before A."

So in an article about ico (indefinite causal order) how do we we know that? That's definite causal order (albeit no measurement has taken place yet as photon in super position traverses these paths).

The ico part must be about measurement... later on ... but i find the write up confusing.