There's no technical evidence (you can check, quite easily, who's peering with who and if there's a cost associated) that L3 and Comcast are using a purely-symm

There's no technical evidence (you can check, quite easily, who's peering with who and if there's a cost associated) that L3 and Comcast are using a purely-symmetric SFP agreement. It's quite likely they aren't (consumer ISPs would never upload enough to large providers to get a symmetric agreement worth their while.)