In a 2-body system this is true, but adding many more bodies to the system makes things very chaotic. An object passing by or colliding with the "proto-moon" c

In a 2-body system this is true, but adding many more bodies to the system makes things very chaotic. An object passing by or colliding with the "proto-moon" could easily deflect it from its crash course. Whether that's probable given the evidence is a different question, but mathematically it absolutely is possible. And that's even assuming that the "mass" is more or less solid - if it were ejected from a violent impact it's more likely that it would be more of a cloud of debris, in which case things get even more complex. So many "bodies" are involved that it becomes more of a fluid dynamics problem than an orbital mechanics one. The individual particles in the cloud interact with each other gravitationally, deflecting each other from their "return-to-sender" orbit. Parts of the cloud would fall to earth, parts would escape to space, and parts would stay in orbit to become a ring system or (as is hypothesized) one or more moons. The theorized situation isn't so much like skipping a stone across the earth as totally vaporizing a big chunk of both the earth and the planet that hit it, and a fairly small part of the resulting cloud (about half according to some models) of debris managing to end up in orbit and eventually collapsing into a single body (the moon).