Upload speed

I received an email stating that my plan’s upload speed has been increased but I must buy one of only five specific modems (none of them Motorola) to get that increase. I have a Motorola MB8611 Docsis 3.1 modem advertised as capable of up to 800 Mbps upstream. Why would that not be sufficient to get the newly upgraded 100 Mbps? I’m still being limited to 20 Mbps. I could have purchased a much less expensive modem if I’d known Comcast was going to refuse to support it to its potential.