Yes, that is correct.
However the requirement that a Mat be non-stacking is only necessary when it actually has Cargo loaded on to it. Since Mats only ‘pretend’ to be non-stacking, it would be possible to add a ‘Mat is stackable when empty?’ option.
We would just have to be careful with the drop phase of movement so that the mat becomes non-stacking before a piece of cargo is added to it. Tricky, but possible.
Another issue I can see in a wider use of Mats in a module, is that there will probably be some Mats that only want to know about some Cargo. I was thinking of adding an optional property expression to a Mat to limit the types of Cargo that are allowed to load on to it.
There will also be problems if you try and drop cargo onto a Mat already in a Stack. I think that would have to be dis-allowed.