You have to understand what a mule is. All ME bronze was intended to be introduced with a modified reverse. The 1925ME halfpennies were (the non-ME 1925 halfpennies used the old reverse); all 1926 farthings were ME and all had a modified reverse. My theory has it that the penny was not intended to be introduced until 1927, with both ME and modified reverse.
However, along came the 1926 penny issue (and my theory about that is quite radical but makes sense) which muddied the waters. It's quite possible the small issue was intended to be completed with the old obverse, but then they ran out of dies - possibly left over from the unexpectedly reduced 1922 issue? - and as the 1927 obverse dies were already there, they decided to use them but possibly under the pressure of time and other factors, they weren't going to produce a special modified reverse for 1926 which was unplanned anyway.
So their use of the ME along with the old reverse means the 1926ME is - AND ALWAYS HAS BEEN - referred to as a "mule". The unique D+d 1926 is what the 1927 penny looked like, and that's when (according to my theory) it was planned to be introduced.
The definition of mule has absolutely nothing to do with rarity.