You are correct. I just looked it up and the general COC doesn't say anything about religion. As pointed out in the video though, it is a violation of US Central Command's General Order #1.
The problem isn't that they are being converted Christianity, it is that the US military is allowing the prostyletizing to go on. What these soldiers are doing could potentially compromise the US
Army's already difficult mission in that region. The war in Afghanistan is not and should not be a holy war and trying to convert the Muslims could strengthen the the idea among Afghans that the US is at war with Islam.