I installed the OP-MTX version together with OP-X Pro III Eco. Indeed the states are compatible with the OP-X Pro II version. I could successfully load an older GP User Preset stored with OP-X Pro II.
However, if you replace an OP-X Pro II instances by the new OP-X Pro III and you had widgets mapped to parameters of the plugin, you will have to remap them as it changed in the new version of the plugin.