That’s incorrect, every 3D program as far as I know allows you to use real-world units, at the very least you can position objects and vertices using units and when you create a primitive you can set real-world values for its dimensions. And it’s very important to do that because things like rendering and physics will work off physical units so if your scale is wrong then many things will be wrong.
What is different is that things like UE4 and 3ds Max/Maya/Blender use 32-bit float values, which means that a position value can only have so many digits, so when something is extremely small or if something is extremely big then it loses accuracy. AutoCAD uses 64-Bit precision which means it can represent very small numbers along with very large numbers. Since UE4 doesn’t use 64-Bit values then it wouldn’t be of any benefit to make something that way. Plus for what you’re doing you will be well within the range to be able to model things comfortably.