First off, Ranked Flex won't be the only ranked system for very long and will be a separate system from Solo Queue.
Second, because it's a separate system, who you get placed with has nothing to do with their rank in Dynamic Queue. That will transfer over to Solo Queue. For now, imagine you're playing ranked in preseason 2/3, before it was changed off the ELO system, when literally a person who was bronze 5 0 ELO in the last season could be matched with the highest ranked Platinum (which at the time was the highest rank) player in their region. Everyone was hard reset to 1200 MMR and the rank of people you play with/against has nothing to do with their previous rank.
GLHF.