That line is a theory (well, hypothesis) about why PPC/EPC/TCDA/etc agents actually have to spend time in the story. I mean, they can read the words on the screen and learn literally everything, so why do they need to go in there in the first place?
Hence, the hypothesis: agents have to make themselves part of the story to change it. In the same way that a long and detailed fanfic can alter the canon (at least according to the PPC... but this is how we get fanon, actually), a long and detailed 'story' - ie, a mission, a linking of event-lines - allows the agent to directly alter the badfic.
It is, of course, Only A Theory (probably out of Analytical Sciences), but it handily explains why some fics wind up being missioned twice: the initial mission wasn't entwined sufficiently for its effect to last. The destabilising effect is proportionate to both the scale of the intervention and its length; we can conjecture some examples:
-Normal PPC mission: duration = 3 (bulk of the story), scale = 3 (the Duty): effect = 3x3 = 9.
-MST: duration = 5 (full length), scale = 1 (just snark), 5x1 = 5.
-Lazy PPC mission: duration = 2, scale = 3, 2x3 = 6.
-Nuke it from orbit: duration = 1 (in and out), scale = 5 (bye-bye world), 1x5 = 5
Throw in the fact that too large a scale can ripple back to the canon proper, and you find the best result is a mid-scale intervention (eg, killing a Suvian), taken after as much of the fic as the agents can stomach.
Which is what we see. :)
But mostly, Morri was just technobabbling.