I tested right-click in your scenario and got from it some very interesting results. My scenario provided for each testing run very similar battle collision, but your test had AI army attack-moving from angle, which gave some variance to results.
In each test run I selected all my units and right-clicked on single enemy unit. On different runs I right-clicked on different units. -23 means that AI units won by 23 an +14 means that right-clicked units won by 14.
Test results: -23, -31, -29, +19, +7, +6, +3, -20, -7, +7, +14, -27
Sorted test results: -31, -29, -27, -23, -20, -7, +3, +6, +7, +7, +14, +19
Based on test results and my observations it becomes clear, that battles generally went one way from the following:
- Right-clicked units got completely obliterated by losing by 20+ units
- Right-clicked units behaved much better in battle and generally won by 1-10 units
In those test runs, where right-clicked units got obliterated, same thing happened every time - they formed a strange flock, which ran around enemy units, seemingly trying to reach some enemy unit, but not succeeding. Clear example of this behavior can be observed from first part of my video in original post of this topic.
In case that right-clicked units did not form such flock and instead spreaded normally between enemy units, they performed much better, like other test results show.
Based on those tests it can be safely concluded, that right-click is designed to behave the same as it has behaved in AOC/HD. Only very serious problem is this strange flocking behavior, which is probably caused by some bug(s) in pathfinding and which will hopefully be fixed 
I could also create and post video presenting different behaviors and corresponding results of right-clicked units, but it is probably easily reproducible by you anyway?
Where did you take this? Right-click has always worked in AOE 2 so, that attacking groups is possible with it. Even in first Wallace scenario narrator teaches new player to right-click on enemy group - “Just click your soldiers and RIGHT-click the red english soldiers to attack”. From there every observant player learns, that right-click works well for grouped combat. This is kindergarten level basics in AOE 2. How could you forget? Too much patrolling probably 
Not only HD. Whole AOE 2 history, 20 years. Core part of AOE 2 gameplay. Test for yourself in AOC scenario editor, if you don’t believe me. Line up 40 vs 40 mirrored melee units, right-click and watch.
I have managed to kill all kinds of surrounded enemy units with right-click. Especially mangonels. Sometimes it happens in HD, that my unit instead attacks some other enemy unit, but then I order him again to attack intended target and then he obeys. It is a minor annoyance sometimes.
I help developers to preserve AOE 2 core gameplay in right-click aspect, which has been in game for 20 years. This allows people to keep on playing AOE 2 in DE like they love. If you don’t like this fundamental AOE 2 gameplay, then you can always keep on playing AOE 1.