Are you referring to Minecraft Pi Edition that came with Raspberry Pi OS back in the day? The one made in C++ instead of the original Java edition?

> run both server and client when in singleplayer - doubles memory use

When did this change? It's been like that for as long as I can remember. It's actually a very common thing for games to do because it requires less code. Also it doesn't actually double memory use - game content can be shared, textures/meshes/audio is only on the client, etc. The chunk and entity data would need to be duplicated but they should be smaller.