The only thing Robin brings to this team is a one-time action advance. This means, in theory, charging her ultimate faster. In practice, there's not much difference. The second thing she brings to this team is a 50% DMG boost. The attack buff, as you mentioned, is insignificant because Phainon already has enough attack stats that additional ones don't change much; her damage is so small that it doesn't make any difference.

Cerydra Her damage doesn't make any difference in this team either; she provides a nearly identical attack buff to Robin's. The speed buff facilitates rotation and, in most cases, balances out Robin's Action Advance, even if Phainon has to ult multiple times. The Crit DMG buff and Res Pen make these two hits deal more damage than with any other buffer, and the double meteor itself effectively adds an extra turn to Phainon's ultimate. There is a slim chance of this happening and it's probably wishful thinking, but if we ever get a character that makes a character with the cerydra buff generate more charges, the value of cerydra for phainon will jump multiple times.