Turns out, the SFP port needs to have speed set explicitly. Steve mentioned this earlier, but I ignored the advice. When I explicitly set the speed to 10G, Comcast said they were getting ARPs, so they disabled auto-negotiation on their side. When they did, the link came up.
So, in the end, an easy fix. But Comcast had advised me to set the link at auto select. Learn from my mistake.