That’s why I conclude they had been instructed. Otherwise, it’s as you say - the eleven were making the decision (or most of it anyway) and ONLY THEN asking the Lord what He wanted.
It’s like someone asking God, “Which would You prefer I use to praise you, the piano or the guitar?” When He’s already told us what He wants: our voices and our hearts.
I don’t think they were instructed because Matthias simply falls off the map in Scripture, never to be heard of again, and Jesus comes along and knocks Saul off his horse and voila, another apostle.
It’s clear that Paul was God’s choice, not Matthias.