Something is different here. Was this a business line? Fiber-based? Something else? Maybe it was because they were performing the installation?
In my case, I initially leased their modem, just in order to get the network up and running. They sent a tech to my home, because they needed to run a coax line from the street to the house. Within the first few weeks, however, I swapped their modem for my own. I went to Comcast’s web site to get the list of what was compatible with my service and then bought one from a local store.
I set up the new modem up myself. I physically connected the modem and powered it on. It took a minute or two to sync up and connect. At that point, my link was redirecting all traffic to a captive portal web page, where I was asked to provide my Comcast user name and password. After providing this information, the modem went into some kind of configuration mode (no Internet connection and blinking status lights) where (I assume) Comcast was pushing down configuration data (and probably a firmware update as well). After a few minutes of that, the modem rebooted and I was fully connected.
Then I phoned Comcast’s support to cancel my modem rental. I drove to their office to return it in person and get a receipt from them, just to make sure there was no mistake (since I’d heard about problems with modems getting “lost”, resulting in billing disputes).