I can't speak for others nor the claims they make, and without knowing when they made those claims and to what specific systems and panels they were referring to, you can't fairly evaluate or criticize them.
As far as shading goes, if shading did not have a demonstrable impact on solar production, SolarEdge would not have a reason to exist since their product does nothing but deal with uneven panel production. Can you cite a recent, reasonably credible source that makes the claim that shading one one cell or one panel will 'shut down' a string system?
Now as for what actually happens and why shading might be an issue, lets take an example of a 10 panel string with 5 panels in full sunlight and 5 evenly shaded so that they get 50% sunlight. What outputs would you expect for a string inverter versus a SolarEdge or Enphase system, assuming the panels have bypass diodes? Which are you most likely to encounter in a real-world situation--partial shading of part of an array during part of the day or one random panel being 100% shaded by cardboard?