I think you have read something in here that NJ didn't actually say. He says the common idea is that those mammals that did coexist with dinosaurs were very small. I can see where 'barely eking out of existence' may be a confusing phrase, but I don't see NJ saying what you think he said.
The sentence is clearly badly written and ungrammatical, to make sense to me you need to insert 'only' before 'very small mammals' and you would have to move some of those commas around.
AS you say it would help if NJ could tell us what he means in words we can understand.