No, the Materials Will Not Count Toward the Damage Inflicted
Let us examine the effect of Chaos Emperor Dragon again, this time with a bit more scrutiny (emphasis mine):
You can pay 1000 Life Points; send as many cards in both players' hands and on the field as possible to the GY, then inflict 300 damage to your opponent for each card sent to the opponent's GY by this effect.
The specific wording of the highlighted text is crucial to understanding why the materials are not included. Importantly, no matter the chain of strategic causality (i.e., "Without activating that effect, those materials would never have been sent to the GY"), it is not the effect of Chaos Emperor Dragon that is sending the materials to the GY.
Any materials attached to the XYZ monster are sent to the GY due to the XYZ monster leaving the field - NOT due to Chaos Emperor Dragon's effect.
Think of it this way: at the moment prior to the effect's resolution, there is an exact set of cards existing on the field and in the hand(s) - this is the set of cards which will be considered sent to the GY by Chaos Emperor Dragon, per the wording in its own effect. Note that any materials on any XYZ monsters on the field are not a part of this set. This is due to the way Yu-Gi-Oh treats XYZ materials with respect to Zone habitance:
Xyz Materials are not treated as being on the field
Again, I cannot emphasize this enough, it does not matter that the XYZ materials were sent to the grave during the resolution of the effect - they were not sent by the effect of Chaos Emperor Dragon. They will not be counted toward the damage inflicted.