I find that hard to believe tbh.
The conclusion if true (as stated in the article) would be that contact tracing would be unlikely to work, because even if you were 100% successful at identifying cases and their contacts and isolating all of them (clearly impossible) you would only be isolating 30% of infected people and their contacts. And perhaps half of that in reality.
And yet the empirical evidence from places like South Korea is that extensive testing and contact tracing does work, and in fact is incredibly effective. I cannot imagine how South Korea would have managed to keep the death toll down to 269 (they have a similar population and population density to us) if they could never identify 70% plus of infected people. That makes no sense.