> According to Stallman, MIT is compatible with GPL but the reverse isn't true
What are you talking about? Let say we have code A licensed as MIT and code B licensed GPL.
You can combine A with B and distribute it. You can combine B with A and distribute it. AB or BA is both a legal combination that a distributor can do. In both cases the distributor need to follow the condition of both MIT and GPL. You can not distribute AB and only follow GPL, or combine BA and only follow MIT.
There is two ways to look at copyright licenses. The slightly incorrect but slightly easier way is to see them as a contract. In order to distribute you need to follow the conditions. If you have two contracts then you have two sets of conditions. Three contracts and three sets of conditions.
The second way, which lawyers prefer as the more "true" perspective, is to see a copyright license as a list of permissions. Copyright law dictate that as a distributor you have no rights, so the license is the specific conditions in order to gain permission. It is like a airplane ticket where in order to travel you got to follow the rules and conditions or the airline. Countries can also have conditions and rules like visas, passports and enter/exiting laws. Thus in order to have permission to travel between countries you need to follow both sets of rules and conditions.
Incompatibility occur when the rules and conditions of two or more licenses demand something impossible, like the case where one condition require an action and the other forbid said action. Both conditions can not be met at the same time and thus the licenses are incompatible.