The single leg romanian deadlift also known as the SL RDL, is predominantly a hamstring exercise. That is not to say it does not work other muscles certainly it does. In fact it does more than just strengthen. The SL RDL will help with balance, and hip stabilization. It will also work the glute muscles and the lower back. It is an excellent exercise to put in a program to reduce risk of knee injury, but equally as beneficial to have in a leg/butt blast program.
In the video and photo you may notice the knee is slightly bent, but it does not bend to the extent that the knees do during a squat. You may also notice at the bottom portion of the movement the shoulders are not much higher than the hips, which is the hallmark of an RDL. This type of movement pattern is referred to as a hip hinging movement pattern.
While the lower back is involved in this movement, that is not where you should feel it. You should feel the back of your legs (hamstrings) get taut when bent over. If you feel this movement in your back, try bending the knee just a hair more, and bring the hips (butt) a little further back. Do not be so concerned with making this movement look pretty. What is important is the back is flat, and you feel the work happening in the hamstring, adjust your form until you feel it in the hamstring.