That was a good argument, but there are some flaws to it. For starters, what does it mean to be sitting on top of a planet? It'd look like this:
http://fdzeta.com/data/MetaMirrorCache/_images_characters_6_81472.jpg
Of course, for Koushiro to be sitting in the zen meditation posture he ought have been smaller than that, not so much though, as you can see for yourself with the conversor I linked this thread.
The size of the room matters not, as the biggest planet on-screen was around ten times the size of MegaKabuterimon, seeing as how he could wrap his arms around it. Even if they were miniatures, it would still be as impressive due to the existance of planets and stars bigger than MegaKabuterimon in such a small room, don't you think? (The size of the room was never stated in the anime, by the way)
If you have watched the episode, the novel and the episode both contradict each other, don't you think? But seeing as how you're using both as one and the same, I'll return the favor. There are two and a half illusions before the main one if we use all sources available:
The first one being that neither Koushiro nor Tentomon could reach the mountain top after climbing the mountain for an unspecified amount of days. I can, however, guarantee you that it was more than a week as Tentomon in the novel cannot recall how many days had passed since they first set a foot on the mountain. Obviously, it was the illusion messing with their perception of space-time. Sounds familiar, don't you think? (And I'm not even going to point it out that they did neither drink nor eat anything while they were at it)
The second one was an anime-only event where they fell inside what appeared to be an endless hole. A mysterious voice (later revealed to be Vadermon) called out to them and doomed the duo to go to hell. The scene was followed up by Koushiro and Tentomon being save by... lifeguards that could float in a space Tentomon could not. That illusion could have very well lead to another illusion, because shortly after, Vadermon brought them to his "Mysterious Universe" of his. Very convenient.
While inside that dimension, their perception of space-time were messed with again. This time, they spent 71 days (without eating and drinking again) inside that place. Strangely enough, Vadermon showcased no signs of being affected by the place, his perception of time was unnafected (because the whole thing was an illusion casted on both Koushiro and Tentomon).
DemiDevimon jumped inside the same hole the duo fell in and didn't suffer from any of the effects they did. Why? Because Vadermon was not paying attetion, causing the hole to be nothing more than a hole that leads directly to Vadermon's hut.
In the anime, no stars were destroyed. the first attack of Vadermon was to hurl a bunch of asteroids on Kabuterimon and then, when he evolved, a relatively large planet (to MegaKabuterimon) and a asteroid field, which MegaKabuterimon destroyed along with Vadermon with his Horn Buster and that was the end of it. We're using both the novel and anime here, so-
Wait. I just noticed something. I can't find the Wakusei nor the Hoshi Kanji anywhere on that game scan, but I can see the modern Tentai! Tentai in japanese means celestial object, it could mean either planet or star. It's the eight kanji in the scan. It makes so much more sense now, it was a mistranslation.
You see, the translator must have assumed those were stars that were being blown to smithereens, when in reality, what was being destroyed by Vadermon were his fake planets. We all know stars can't be "blown to smithereens", but planets can. "Then why he could identify that Izzy was sitting on top of a earth-like planet before?" It's rather simple: first of all, Izzy couldn't be sitting on top of a star, second and most important of all is the description that the celestial object receives.
To confirm that I need the original japanese novel so that I can see what Kanji is used when referring to the stars.