Comcast is a SolarWinds customer according to some material SolarWinds published ahead of the breach [1]. But it is unlikely used to manage customer equipment like modems. So far, I have not seen anything suggesting that Comcast was affected by the backdoor.
About 18,000 SolarWinds customers downloaded the backdoored version of SolarWinds. Out of these, about 1,000 had the backdoor used (based on passive DNS data). I did not notice XFinity/Comcast on that list [2]. For most customers, personal information like credit card data would be at risk. But in the worst-case, an attacker could manipulate cable modem settings/firmware.
In some ways, Comcast, just like SolarWinds, is a supplier that could be used to attack you. I wouldn't trust Comcast to deliver lunch without stealing a couple of fries, or drop the soda and trying to scoop it up to hide the fact. Everything outside my firewall, e.g. the Comcast modem, is considered hostile territory. No Comcast equipment or Wi-Fi is "inside" my network.
[1] https://web.archive.org/web/20201214090828/https://www.solarwinds.com/company/customers
[2] https://blog.truesec.com/2020/12/17/the-solarwinds-orion-sunburst-supply-chain-attack/
