That is certified for xfinity which is the same as comcast. The only way that comcast will refuse to allow use of this device is if it was flashed with a hacked firmware that gives bandwidth controls to the customer vs just comcast. If you never flashed the firmware in this and its still the firmware from Netgear then whomever your talking to might not know their job or they are trying to get you to lease theirs for $6 a month.
I have had my own modem for about 6 years now and recently went from Docsis 2.0 to 3.0 upgrade in which i had to replace my modem with one that was docsis 3.0 compliant as well as it has to be xfinity certified. I had no problems once I got comcast on the line after the 45 minutes on hold waiting for assistance to give them my account info, confirm my identity as the account owner, and gave them my MAC address. After they flashed settings to your modem it will reboot 1 or 2 times and then you should be golden.
