This is correct.This is my understanding as well from the reading I've done.
My understanding is also that since he is the one wanting to adjust for it, he's the one that needs to provide full financial disclosure for me/my lawyer to review.
Family Law Act - O. Reg. 391/97
This is Schedule II from the Child Support Guidelines. It is the "Comparison of Household Standards of Living Test." This is what the law requires to compare households to determine if there is a case for undue hardship. The formula is not that complicated and you can do it by hand with a calculator in a few minutes if you have all the required information.
If the required information is not provided, then there is no undue hardship. A court requires this.
In your situation, your ex is asking for a personal agreement for undue hardship but not providing any information. In this situation your answer is simply "No." You cannot be forced to sign any agreement. You may sign an agreement if you choose to.
The risk that anyone runs if they choose not to is the question of whether they were being "reasonable" in terms of a court awarding costs. If you think you would not lose in court, then you have nothing to worry about. If you think your ex would receive a decision in their favour, you are not at risk until the point where they provide full disclosure. Up until full disclosure you cannot be expected to agree to anything.