The problem is with the way the decision was made. There’s no indication that it was Spirit lead but more likely a man made decision.
Giving God two *choices* and then casting lots, isn’t much of a choice.
Someone was going to *win* and someone was going to *lose* but that doesn’t mean that either of them was who GOD wanted chosen.
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.