In the interests of being totally accurate (even pedantic), I don't believe that CB penetration occured, but that the poster flew underneath.
Of course, I don't know what the cloudbase was, as it could be anything from a few hundred feet to a few thousand and thus the ground clearance implications of downdrafts are unclear in this context.
But put it this way, I would not do it ... and I have inadvertently penetrated an embedded cell in a light aircraft, which was one of the scariest experiences of my life. And we got away very lightly.