how is this different from lvgl? is this immediate mode or retained mode?

Very different, starting at the line count:

Lvgl is 440kloc across 1134 files (in the src directory), while microui is 1121 lines of code in one .c and one .h file.

Microui is immediate mode, very minimal and 'bring your own renderer'. Probably most useful for adding a small debugging UI to a 3D game/app.