Xfinity only allows 35 Mbps upload, but my modem supports 800 Mbps up (plus 2.5 Gbps down). Why is Xfinity artificially capping my speeds?

As in the title. I have a modem, a Motorola MB8611, which can do 2.5 Gbps download and 800 Mbps upload. However, Xfinity says it only supports 35 Mbps, which is completely wrong, by a literal order of magnitude and then some - it literally supports over 20 times the speed Xfinity says it does, and 8 times the speeds I expect.

When I upgraded, the Broadband Facts and the representative both said I would get 100 Mbps upload (which is still just a fraction of what the modem can do), and I made sure when I bought this modem that it supported those speeds. That being the only reason to upgrade, I expected the full upload speed, but it seems Xfinity is artificially capping me because... they don't like my modem, or something? They want me to buy theirs and have decided to act in subterfuge toward that end? I don't get it. There are modems on that list with a fraction of the bandwidth of mine, and Xfinity allows those a 100 Mbps upload speed, yet mine is left in the gutter for some inexplicable reason.

I get that actual traffic, bandwidth, network behavior, etc. will change and depend on network conditions, but I expected and still expect that Xfinity at least tries to allow my hardware to do what it says it can do rather than artificially cap my speeds because they think I don't know any better. This is completely ridiculous, and completely dishonest on Xfinity's part.

Anyway, my neverending complaints with this godforsaken company aside, is there a fix? Thanks.