They don’t know either. They’re just guessing.
Yes, the moon is known to have lava tubes; this has been known for years, in fact, prior to the manned lunar missions; image search:
https://www.google.com/search?q=lunar+lava+tubes&tbm=isch
the trick is, to find one that hasn’t collapsed:
http://www.lpi.usra.edu/decadal/leag/AndrewWDagaFINAL.pdf
The article BenLurkin has posted here is a study of how large these tubes could be and still be stable and usable for a habitat.