On top of that, Missa Dei Filii is not a full-blown catholic mass. Zelenka may have intended to expand it, but then again he may not have wanted to for some reason!
This has always interested me. I have come to conclude that, to be a completely devout Catholic did not mean that he did not have to take the most obvious Catholic way in his writing. To be a strongly religious Catholic did not mean that one could not take an interest in a more Protestant method. I think that he appreciated the Missa Brevis style, and wished to dedicate one mass to that, as a small nod to it perhaps? No evidence for this whatsoever, but I am going purely by whim.
Just as Bach in the end wrote in a very Catholic manner for his mass in B minor. (not that bringing Bach's example into this makes this debate any better)
Plus, of course it is not 100% whether Missa Dei Filii -is- a brevis.
It is in any case a fascinating thing to dwell upon.
Seb.