Scott was not intentionally trying to speak of Jean in the past tense so much as she was so newly back to life that it was hard to get out of the habit of speaking about her in such a capacity. There was also a difficulty in expressing his thoughts in a way that reflected her and the way things had been as compared to the present, awkward and on the verge of implosion once his revelation was put out in the open.
Jean had never, in all her life, pulled away from him the way she had when he spoke Emma's name and laid out the relationship for her. Never spoken to him with such malice either that he could remember and her words might as well have been daggers for the way that they fell on him and yet... he could not bring himself to entirely blame her. He had hoped for some semblance of understanding but been prepared for the worst. It was hard to meet Jean's gaze, even behind his ruby quartz, but he managed all the same and fought the urge to stand as he felt that would only elevate things further. At least if he was sitting, she could be over him and not the other way around.
"She's not the woman you knew her to be, Jean." Defending Emma likely was not going to help matters but he did not want Jean to believe that he had somehow been won over in her absence. "I know based on what you remember of her, what your past interactions have been, that you would feel otherwise. But she was good for the school... and she helped hold me together."
It was clear Jean did not like what he was telling her, but he had resolved to be nothing but honest. That was the only way to move forward for either of them.