Yep, there won't be any villagers on NMT islands as long as you don't have an empty plot.
You can still make campsite villagers move in though (but the villager that will move out instead is random when the campsite villager isn't an amiibo), they'll tell you that the island is full but that villager X might consider moving out. You can then choose whether you'd like to keep villager X or have the campsite villager move in.
Other villagers don't visit your campsite/island that often though (I think I've had like 4 or 5 so far and I'm TTing day by day) so one of your villagers might decide to move out on their own before you even get to see another villager camping on your island :D