I lifted the follower and peered in from the opposite end of the top bar hive and it looks like have already started to draw out comb. They were out of space, so that explains why they started to get to work on the top bars so quickly.
To help improve communication without the spacing issue, I might make a some top bars that have a few inverted Y shaped tunnels drilled in to them. That should let the bees still use the bar for comb and provide a channel for them to move from top bars to the NUC. When I eventually remove the NUC, I could either plug the holes with cork or let them use those as additional entrances down the length of the hive.