I wouldn’t call it a good article. When dropped from above knee height? That is absolutely nonsense! If you drop a helmet from the height of a motorcycle seat (for example) then it is unlikely to need replacing, after all it can take an impact with a head inside it which would be more than an order of magnitude more damaging. An empty helmet wont have it’s impact absorbing foam compressed during a drop and therefore wont have invisible damage done.
The results of a google search: https://www.google.co.nz/search?client=opera&q=do+I+have+to+replace+a+helmet+after+dropping+it&sourceid=opera&ie=UTF-8&oe=UTF-8
When someone selling motorcycle helmets says you should buy a new one every time you drop it, I think of how good their profits would be if all riders followed that advice.