Yes that’s true, no lights are visible on the plane, but on the flight manual the switches on the triangular overhead panel are well described, as you can see on the attached picture.
There are dimmers for instrument lights, panel flooding and exterior lights as navigation and anti-collision as well. Looking on the plane though, where one would expect to see the lights, only some possible supporting plate is visible. I guess the lights were removed for some reason, or maybe not installed on first prototype ?
On this page
I found some very detailed close ups of the fuselage; in one of the last images you will be able to see a circular plate with the sign “auxiliary landing light” exactly where it was supposed to be according to the XB70 manual, but apparently it is not there.
