Not only Comcast uses Level 3 when trying to connect to SE's servers.
AT&T uses them, and, as Kosmos said, so does Charter.

But SE probably wouldn't just point the finger at Level 3 directly. So they use the blanket "certain/several ISPs" instead.

SE isn't at fault here for the connectivity issues.
However, I am still miffed about their slow response time. Took them nearly 10 hours to even make a mention of this. For that, I place the blame solely on them.