Maybe because he already had the 3D doo-dad beforehand so it warranted mounting in the TS. But the stylus setup looks awkward to my eye. Now it also has to be vertically on center & the TS is no longer available for any longer part to copy. Maybe just a short part run trick for a few artistic shapes, which is totally fine. They look nice. Another approach is a 2D template located to the rear of the lathe & stylus mounted to cross slide. Industrial versions even had hydraulic feed.
It is important that the stylus & cutting tool match exactly for the entire range of cutting profile. Maybe my eyes are playing tricks but in the video it almost looks like his hockey stick stylus might give a false contact read if the angle shank contacts before the cutting tool tangent.
There was another gadget, I think Blondi was plugging it at some point. Kind of a hand held affair that slid on a platen on the cross slide? It appeared to work with very light cuts on brass but I value my 10 full length fingers LOL.