I've located my old lens tables booklet that was published by Zeiss Ikon for the Carl Zeiss lenses.
Here's what it has:
When focused to 4 feet, at f/1.5, the depth of focus for the 50mm Sonnar is is 3'11" to 4'1-1/4" -- roughly 2-1/4 inches.
With the 85mm Sonnar focused to 4 feet, depth of focus is 3'11-1/2" to 4'0-1/2". That's just one inch, and that's not much room for error.
Possibly, you could expect similar results with the modern-day lenses, but that's just a guess and I'm not an engineer, so I could be way off.
If the actual focal length of the new C-Sonnar is longer than the actual focal length of the old Sonnar, it's possible that depth of focus would be shallower.
Still, I don't think it would reach zero, and perhaps Mr. Scherer is simply taking some editorial license in his argument that depth of focus is extremely shallow.
In the end, the choice is between old and new. Old Sonnars have a certain look. The new C-Sonnar has its own look as well. It's not "which is better?" but "which do you prefer?"
That's how I see it.