What makes it credible is that there are other second century writings quoting the Long Ending.
But there aren't are there - as far as I am aware there is no extant copy or fragment of any of the texts you mention that is dated to the second century that contains the long ending. Maybe you know different - in which case please share.
What we have are much, much later versions, often translations which purport to be copies of documents originally written in the second century. However we do not and cannot (unless an earlier copy turns up) know whether these versions from the 4thC through to centuries later actually contain text which was in the original.
Realistically I'm unclear whether there is any extant fragment or document from earlier than the earliest version of Mark itself containing the longer ending - which I think is the 5thC Codex Alexandrinus. If the longer ending had been added by then (as we know it was) then it isn't surprising if, in the interests of orthodoxy, references to it start appearing in 5thC versions purporting to be versions of works by the early church fathers.
