CMS are correct here.
50% is half. That means the costs incurred by the other party is half of the whole and half is your responsibility.
The incomes of each person do not matter in this regard because nothing is due.
If say you are jobless and your ex is rich, it still doesn't alter the situation. Their money is theirs and yours is yours.
If you can't afford to have your kids 50% of the time and pay for housing and food, then you have to give up some of that time to the parent who can afford to provide such care.
I had similar arguments with an ex partner, even though I moved hundreds of miles to be closer to my daughter, bought a house and got 50% care and thus had my own costs, they still wanted to receive child maintenance just because they earned less and they needed childcare on their days. But here is the point: their childcare costs are not mine, my childcare costs are mine. I am not responsible for childcare costs on their days because the care is shared 50%.
I even said "I'm not refusing to give more money if they need clothes and whatever, just send me the receipt" - all vehemently argued against but all was false. 50 is HALF, nobody owes anything to anyone.
In regards to the mortgage: You have to tell the mortgage company your outgoings and costs. If you have 50% care, then you have to state the expected costs of that care because it is an outgoing by someone dependant on you. It doesn't matter if it's informal or formal.
But here, since there is no payment to either person, you only need to state your costs not payments since there should be none to either parent.