I think that's making the assumption that having 100 billion livestock + the current human population is sustainable. Even if that were the case, humans consume a lot more resources than animals living on factory farms.
I think the point was that if humans were vegetarians we wouldn't need 100B livestock. And since a cow needs quite a lot more food than humans do, if we could transition 1:1 to a human-edible crop then we have plenty of food to support humans.
Whether or not the earth can sustain 100 billion humans was the scope of the statement, so I don't think resources other than food is a separate issue.
I agree with the assertion that humanity is more sustainable (all other things being equal) vegan or vegetarian rather than omnivorous, but it's going to far to suggest we could just swap in humans for livestock.
