I was speaking in generalities. Logic. Science. Facts. Take your pick.
An outcome cannot be certain when an input is presumed.
Presumed=converted to morally certain.
To say intention is presumed is not
to say "well, I'll never know, I'll just presume
he intends to make a sacrament happen." To say intention is presumed is to say "Given that men act as they think, and this man is acting in the only way by which it is possible to confect a given sacrament, I presume
to act this way, and therefore am morally certain that he has the required intention
But to take the "invisibility" of intention and, from that, conclude we cannot be (morally) certain
about intention is wrong. If we are not
morally certain about intention, then we can't partake in the sacrament.
The reason that the language is that we presume
intention is that intention is internal. We can actually see/hear
that matter and form are correct and present, so there is nothing to presume
in regard to those elements. We cannot see
intention, we can only see indicators of it (mainly, the presence of matter and form correctly applied). But in the presence of those indicators, we presume
that the intention is there, and that presumption itself
is controvertible with moral certainty. So don't let the language throw you off.