Why can't we do flex with anyone now that we're all unranked?

I don't understand why I can't do flex rank with my friend who was higher rank than me last season. I mean, I get why I can't because he was gold and I was bronze, but now that the season restarted, aren't we supposed to be all unranked? At least Riot should edit the message, so it shows "you can't play together because of different MMR" not because of different ranks.