There is no entry for MOTLIER, MOTLY or MOTLIE on en.oxforddictionaries.com.
The entry for MOTLEY doesn't mention even that it has a comparative form at all (which would be required as it's more than one syllable), let alone that the comparative has the irregular form MOTLIER.
The most recent occurrence seems to have been just under a month ago. I can't believe that, all the times it's been used, not even once has a correct verdict been given. So I can think of a few possibilities:
- it has been removed from the dictionary very recently
- there's a rule we don't know about whereby if a search for the word returns something, then it's considered valid, even if the entry itself doesn't make it valid in any way whatsoever (I'm not sure whether we've seen evidence that contradicts this)
- it goes to prove (as has been suspected before) that they're using a different dictionary from the one everyone thinks they're using