Yup it's capillary action plus the 'sucking' as plants allow excess moisture to evaporate through the stomata concentrated benteath the leaves.
You can grow plants upside down but with cannabis it's not worth the effort and could even be a source of stress. An exception would be the Omega Garden, which actually cycles them through right side up and upside down like a ferris wheel that doesn't have free-swinging seats, lol.

There is an installation at the Mass MoCA that is a line of small sugar maples in planters suspended upside-down along the walkway to the main doors. Very cool, but as art, not a legit grow technique.