I think they specifically mean Wi-Fi, not confusing it with cellular.
I would be surprised too if I went into a cave and suddenly my phone is connected to Wi-Fi. Where are the access points? Who is running internet all the way out here? I don’t see any telco boxes or signs of a human presence. Why do the speeds get faster the farther I go? Why did my phone connect to some random network when I specifically told it not to do that? It would be even weirder if it’s WPA3 protected or something because I obviously don’t have the password for the cave Wi-Fi