It's not about Comcast slowing down internet for AT&T users, it's about Comcast slowing down the AT&T website so that people using Comcast (especially in the many many areas of the US dominated by a single ISP) will have a harder to switching. Or, more likely, it'll allow them to reduce the ability for any new potential ISPs to get off the ground.

This is just a worse case scenario, though. It's important to keep in mind that none of the big ISPs are gonna pull anything like that for a while, especially since so many states are strongly against the repeal and will try to keep net neutrality.