I am rather confused and my imagining is that they are more concerned about how much disability you were paid by the 3rd party (which might have disqualified you, your income would be different/sufficient (according to them) without SSDI? If you were getting a "paycheck" whether through working or through employer disability, I imagine it is kind of like when you qualify for COBRA insurance after you leave a job and have some other insurance too/instead and they won't pay you because you have or could have/qualify for COBRA? That sort of thing.