ubnt has been the ubiquiti default login at least back to 2010 when I started using their products, before UniFi was a brand. I always assumed it was short for Ubiquiti Networks.
A lot of companies in that space did then. I was at a robotics company at the time and we experimented with mikrotik routerboards + the various long-range Ubiquiti wifi modules, some of which are even still listed on the website: https://techspecs.ui.com/uisp/accessory-tech/xr (though not the 900 MHz XR9, which was arguably one of the most interesting for long range comms)
