LuCI provides a fallback emulation for uhttpd-mod-ubus. Upon loading, luci.js will first send a test HTTP request to /ubus/ to see if it exists. If the test requests result in a 404 error, the ui will fall back to the Lua emulation at /cgi-bin/luci/admin/ubus.
I do plan to make uttpd-mod-ubus mandatory though as it is significantly faster than the emulation.