In my experience, Comcast shit never shows up on your credit report. I have had this exact same experience and got no where arguing with them so I just ignored it forever and nothing happened. This was over 7 years ago.
If it does hit your credit, you can dispute it and there’s a decent chance you’ll win. I’ve had that issue with direct TV where they claimed I never returned equipment I had returned. But these companies get in to this situation because they are terrible at record keeping.
Also, any time your speed dips below advertised, file a complaint with the FCC. They won’t get in trouble but it will annoy them enough you’ll get fast tracked customer service. I used to have some VPs phone number and I tried for like 40 minutes to get her to just admit to being evil. She wouldn’t. But they changed the phone number because I posted it everywhere after I stopped having to have Comcast or I would give it to you.