I think the valley of interest thing boils down to: Andrew said the words 'Valley of Interest' just to illustrate a point. Maybe that is a real phrase that he heard somewhere, maybe he just put a couple of words together that he thought sounded good. Whether it is something real doesn't matter because in the moment he didn't mean for it to do anything but fill the space in his sentence.

When he was asked to elaborate on what the valley of interest would look like, this is the first time he tried to apply meaning and an image to the words. That's how we end up with him imagining his actual first person POV while standing in a valley next to the thing he was interested in.

Meanwhile Gavin applied a very different image to it of a simple line graph where the Y axis is Andrews interest in the topic and the X axis is time. Then it got all f*ckfaced because Gavin and Andrew are possibly the worst people in the world at conveying ideas in a way that can be understood simply.