Great comparison. I think the confusion is caused by the graphic on the front panel of the BB800. The graphic implies it might work as a HPF when turned to the left, but I think that might be more of a visual shorthand cue (like an emoji on your smartphone) rather than an accurate frequency graph. If I had to take a totally random guess, based on how I think Pat Quilter's brain works, the 'Depth' control probably has at least EQ two bands, I'm guessing maybe variable shelving at 100hz and fixed HPF at 30hz? and Pat has carefully studied the "desirable" sonic qualities of vintage tube amps.
When I compare my Broughton always-on HPF to my BB800's Depth control, my ears hear something a bit more "surgical" happening with the HPF. My favorite EQ setting on the BB800 is Depth 10 o'clock and Contour 2 o'clock. This gives a nice midrange-forward 'vintage' tone that sits well in the mix and doesn't really need a cab sim.