Can furniture be placed in the open world?

The word doing the work is "certain". This is not the whole homeland catalogue travelling with you. It is a subset, the interactive pieces, and they can be deployed and used away from the homeland rather than just looked at. The point is the item doing its job somewhere other than where you built it.

That matters because the open world is where most of the game sits. Every creature in Aniimo can be encountered and captured in the wild through exploration, so there is no part of the roster you fill in from home. Umbral Aniimo, also called Umbra Aniimo, spawn naturally and at random out there, so a run can turn into something worth staying for without warning. Aniimo eggs are hidden across the map and picked up as items. Environmental puzzles are scattered through it as well.

Add those up and a long trip away from the homeland is normal play, not a detour. Being able to set an interactive piece down in the field is what keeps that trip from being broken by a journey home. Plan around it the other way too. If a piece you rely on is not one of the ones that travels, you are going back for it.

Not settled: The set of furniture that works outside the homeland is a restricted one, and which pieces belong to it is not settled pre-launch.