What are the odds OU/TX asked the B1G and were turned away? Versus the odds that was a rumor unfounded?
More likely just a rumor.
I think it sort of depends on what we mean by "asked the B1G and were turned away."
If we're talking about "made overtures through back-channels as much as two years ago" then I'd expect that it absolutely did happen. Clearly, all of this has been worked in the background for a very long time, which is what it takes to make a conference move like this. I'd be shocked if Texas and OU didn't explore options with each of the two premier conferences.
And given what we know about some of the higher-ups at both OU and Texas in the recent past, who clearly desired the B1G for academic purposes, I'd think those were very serious overtures, not just some low-level Q&A bulljive. I think both Texas and OU would absolutely, seriously, have considered the B1G, once they made the decision to leave the B12.
If my assumptions above are true (and they may or may not be), then this would suggest that either a) The B1G entertained the overtures and decided against it or b) The B1G was receptive, discussions proceeded at least somewhat down the path of submitting for membership, but that ultimately Texas and OU decided that given both options, they preferred the SEC.
I could see it going in any direction, but I don't think I'd dismiss the idea that B1G leadership took a pass on the option, given that we KNOW OU and Texas were serious about leaving the B12, and we know that in the past, factions within OU and Texas have had a strong preference for the B1G, because of academics.