Yeah, I saw all of that (and more). Doesn't rule out what I said.
I presume Comcast was advertising those (longer) prefixes to Level 3 to manage traffic flow and they shouldn't have been propoagated to other customers. To do that, you'd typically apply BGP communities (no-export or other, Level3-specific ones) to those prefixes. A lack of those communities on the prefixes would result in them getting propagated to other Level3 peers. When that happened, it would look exactly like and result in this route leak.
I presume Comcast was advertising those (longer) prefixes to Level 3 to manage traffic flow and they shouldn't have been propoagated to other customers. To do that, you'd typically apply BGP communities (no-export or other, Level3-specific ones) to those prefixes. A lack of those communities on the prefixes would result in them getting propagated to other Level3 peers. When that happened, it would look exactly like and result in this route leak.