This is just a way of saying that we have probed the electron for internal structure to a significantly more precise level than the level necessary to reveal th

This is just a way of saying that we have probed the electron for internal structure to a significantly more precise level than the level necessary to reveal the known internal structure of nucleons. But it's just an upper bound. For all we know, the next accelerator (if there is one) could reveal composite structure just below the bound.