Well, that sort of highlights my issue and why i assume they meant mobile data. If they are, in the joke, connected to WiFi, then they wouldn’t be surprised by the signal strength. Because they are near a router and therfore the signal is good enough to use. You aren’t typically able to connect to WiFi if the signal is below -70 -80 decibels.
I think its fairly clear that they meant mobile data as this is something you would be surprised to still have if you were lost in a cave.
Not that i wouldn’t be surprised to find WiFi down there, just that i wouldn’t be surprised by the strength of the WiFi signal if i was standing close enough to a router to see the SSID.