OK, I quickly skimmed through the reasearch paper without going into the math, but here's the skinny of it.
They used 2 WiFi routers with 3 antennas each as cheap makeshift radar. Router antennas aren't designed to natively provide elevation and angle information so they had to get smart with the data processing. Once they have the data from the antennas they used cameras to train a proven AI model for recognizing human poses and mapping them to a 3D mesh on said data. They switched to 15 different room layout and proceeded training their model. Then, they switched to a new untrained room layout to test the models performance. The results were always below image based recognition and plummeted even lower after switching to an unknown room layout.
Unless it's buried between the math paragraphs I don't see "looking through walls" mentioned in the paper. The introduction section has a quick mention that visual obstacles provide difficulties for other human recognition technologies. Unless it's because of the implication of WiFi going through walls, I can not discern where this article got that idea from. The superimposed example images in the research paper even cut-off at the legs if the person happens to stand behind a table.
My takeaway from this is, as long as you don't make the specific placement of your multiple WiFi routers and the exact layout of your house public knowledge and don't set up multiple cameras with overlapping views to cover every angle of your home, you should be safe. Or just get single antenna routers.