Why is my symmetric gigabit plan capped at 100mb up

I went to check my plan and saw that there was an option for 1GB up and down, but after selecting and applying the change I looked at my plan details and I'm capped at 100mb upload, where I was at 1gb/300mb before. I have a Hitron Coda56 modem, which advertises up to 1GB up, and Xfinity has certified for up to 455mb up. I was contacted by a sales rep via chat window who claimed that it was capped due to "user equipment compatibility" and then referred me to their "technical support" line, which was just the standard 1-800 robo tree. Why is my plan being artificially capped at such a disparity from the advertised product?

Attached screenshot of the plan builder display because I haven't been able to load my plan details page since the live chat.