mmmmm not the way I see it, the channel manager I have sends a set price and then airbnb follows you set with regards to extra people. I know this because I have 2 separate listings, one with an increase for guests and one without. The same price is being sent to both listings.
The one with the increase per guest set has a base rate of say $100 for 1 or 2 and $150 for 3 guests. The one with NO increase per guest set has a price of $100 for 1, 2 or 3 people. The base price gets sent for the room and airbnb follows the rules you have set, just seems that you cant change those rules after the channel manager has been added.
Now other channel managers might be different, and please tell me if they are, but in this case this is how it seems to be working.