Thank you for the link. Your statement is that the combo makes them more susceptible. Your reference does not say that. It says they cancel each other out, they don't say it makes it worse. That is two different things.
Freedom Ranger's comment is wrong anyway. Medicated feed does not contain any strain of Coccidiosis. Medicated feed contains medicine. In the US that medicine is almost always Amprolium but you can find some medicated feeds with different medicines. And while the medicine can cancel out the vaccine, the vaccine does not cancel out the effects of the medicine. The medicine still works.
The exaggeration is yours, there are enough myths and misinformation floating around on this forum, that's why I spoke up. The misinformation is on Freedom Rangers. I'll try to contact them and ask them to confirm that with a veterinarian.