Situations like these suggest that there's a lot of improvement to be had left with the matchmaking system, for sure. I cannot say this with full confidence, but from the looks of it it seems like matchmaking autofills players into a role before placing them into the lobby, which leads to these kooky situations where everybody gets autofilled into the wrong position, despite nobody needing to get autofilled in the first place.
At this point I think Riot needs to acknowledge that getting autofilled is a very bad feeling in itself, and should only be used as a last recourse when a team really needs a player in a role that cannot be easily found. Getting autofilled for no reason, as was the case in the OP's situation, I think feels especially disrespectful of the player's wishes, as the latter is being forced to change their plans and potentially play something they don't enjoy with no real justification behind it, besides some quirk of the matchmaking system. If the intent is somehow to get players to try out new roles, that is something players should either decide on their own or be incentivized to do through missions, not something players should be forced to do, especially in ranked play.