The API mined data from Eternal Palace (which was the last raid before Blizzard shut off the API stream) looked like this:
40% of players had Ahead of the Curve (cleared Heroic Eternal Palace)
60% of players cleared Eternal Palace on any difficulty
This means that the %% of people clearing Palace on Normal, or LFR, but NOT Heroic, was 20%.
And 25% of the players killed the first boss in Mythic.
So you are absolutely correct: More people did SOME Mythic raiding than did ONLY LFR. (And even then, those LFR numbers also include people who just did Normal, since there’s no way to tell them apart via achievements.)
In this case, the sign was originally made and corrected prior to the taco truck opening for business. No one ordered any tacos with the assumption that they’d get chips with it because the sign was corrected and visible to all.
Further, I expect that no one has subbed to this game so that they could get that mount by doing LFR content only. So no harm, no foul. I don’t really have an opinion on whether the mount should be available in LFR, I really don’t care, but all of these analogies assume people invested something that they’ve now lost and are now worse off because of the error.