So, in this case, the lens blur they are talking about happens if you stop the aperture down too far.
To clarify further, the aperture is an iris inside the lens that controls the amount of light entering through the lens. It also controls the depth of field which is how much of image is in focus at the same time. The maximum (wide-open) aperture value also determines the speed of the lens.
The smaller this value is (expressed in f-stops), the faster the lens is considered as it has a greater capacity to gather light and focus it on the imaging medium. An f/4 lens is slower than an f/2.8 lens, which in turn, is slower than an f/1.4 lens, etc. The faster the lens, the shallower the depth of field is wide open.
Now if you stop a lens down too far, say > f/11, the lens will begin to lose sharpness due to diffraction. The new feature in Rawtherapee attempts to correct this.
I will take some test shots in the next couple of days and try out this new feature as Manjaro just updated my Rawtherapee to 5.8 today.