Why do the Comcast IPv6 test pages pass for me with 38em as shown below.
That is because the router still acquires, and advertises, a prefix delegation and DNS servers. Comcast prefix delegation address, like yours in the screenshot, start with 2601:[...]. While their host IPv6 addresses start with 2001:558:[...]. The latter address is what you will see under the IPv6 WAN address field.
The reason this isn't an issue for most people is because your IPv6 WAN address would only be used for connecting directly to the router from the internet via IPv6. A couple examples would be connecting to the OpenVPN server on the router via IPv6 (which isn't currently possible), or ssh into the router via IPv6.