I call BS. The federal convention recognized that a secession could occur, and Madison offered a motion that the militia could be used to prevent secession. That motion was defeated 8-3.
So far as I can make out the measure was tabled or indefinitely postponed. So the convention never made up its mind about the matter.
It's strange, though, that after something like ten years arguing this, you bring this up now. If it were all that essential, wouldn't we all have discussed it by now?