That situation isn't helped by people going on here about how they'll make their own decisions about whether someone else is medically fit to dive, when they aren't qualified to make those decisions. For example, T1D isn't epilepsy, and someone with hypoglycemia isn't likely to manifest that by suddenly and unexpectedly convulsing. Responsible T1Ds following DAN guidelines can dive. I suppose the concern for instructors is whether a given person is responsible, because some certainly are not, and no one wants to have to recover the body of a foolish person.
You're touching on something important here: it's not just about whether or not a doctor believes a student's medical condition is compatible with diving. It's a combination of the medical issue, how responsibly the student handles it, and how well the student and instructor communicate so the instructor feels that the medical condition will not create an unnecessary risk.
I've had one student, several years ago, that was Type 1 diabetic. He was 19 or 20, and he brought me a signed medical form. We discussed all the details of the situation, and his parents joined the discussion. We made a plan A, plan B, plan C for the unlikely event that anything should go sideways with his blood sugar while underwater. We spent a day on a dive boat and he made a plan beforehand, which he shared with me, on when and how he'd check his his blood sugar over the course of the day. Overall, he was 10 out of 10 when it came to the "responsibility" factor.
And of course there were no problems. He even came back for his AOW course a few months later.