Another good question it would be helpful for Fdev to clarify. The way the addons were piled into Horizons suggests so, or alternatively it may simply be a lack of forward planning. I think any future DLC may well require elements of Horizons (passenger cabins, guardian stuff etc) but the single current DLC will just be made cheap enough that it overcomes the issue of compatibility.
otoh base building can be in space as well as on planet surfaces, and a player doesn't need to be able to land on airless worlds to be able to land on atmospheric ones or to walk around the ship & space stations.
I would have thought we would know this level of detail of the roadmap by now tbh, is the LEP an obstacle to this either way?
It might be that you can't get up full stop, or do anything on foot or join a non Legs crew. Time will tell.