And if it was just left at that it would be a pretty terrible use of illusionism, and I'd have to question why the GM included the rumour table in the first place if they didn't intend to do anything with the rumours.
But let's suppose I want to do this exact thing for some reason. Maybe I intended to prep for both orcs and spiders, but my pet iguana ate the MM pages containing the rules fore spiders so now I have only the orc encounter prepped. Or perhaps I'm just a nefarious power-mad illusionist. Anyway, as the characters arrive to the Gnarly Forest they come upon several slain spiders, and if they investigate they manage to find some arrows of orcish manufacture. But there are no orcs in the Gnarly Forest, and the closest orc settlement in the Grim Chasm is a significant distance away! What are the orcs doing here? This is not just some random orc raiding party, the spiders have nothing of value. Has someone send the orcs here and to what end? So now instead of just one random encounter with orcs on the orc territory, the encounter will now be tied to an emerging sublot. The actions of PCs did affect things, just not in the way they may have anticipated.