MMR Dropping: Losing in arena on purpose to lower own MMR below normal value.
From the first thread that you linked:
I got banned for MMR Dropping, but I never got my MMR above 0.
This is incorrect since MMR starts at 1500 not at 0. So by only losing, he would be below the normal value of 1500 and would technically be breaking the rule.
I believe the rule was meant for somewhat competitive arenas, to prevent people from getting higher rating by starting at lower than normal MMR. Normally MMR dropping would be used if a team or a player wanted to start a new team at normal MMR, instead of starting it at high MMR and getting high MMR opponents from the beginning.
I am not sure if the rule is really supposed to be applied in this case. But he is also just leaving10 arenas instead of playing them and getting arena points for doing so, which could also be seen as abusing the system.
If you want a more specific answer, you will have to send a private message to Head GM Aldtharios.