Wall-clock usually refers to time actually taken, in practice, with the particular configuration they use, not time could be taken if they used the configuration to minimise start-to-finish time.

I suspect hobbyist experience isn't relevant here. My experience running workloads at large scale (similar to Pixar's scale) is that as you increase scale, thinking of it as "enormously parallelizable" starts to fall apart.

Ray tracing is embarrassingly parallel, but it requires having most if not all of the scene in memory. If you have X,000 machines and X,000 frames to render in a day, it almost certainly makes sense to pin each render to a single machine to avoid having to do a ton of moving data around the network and in and out of memory on a bunch of machines. In which case the actual wall-clock time to render a frame on a single machine that is devoted to the render becomes the number to care about and to talk about.