I am not certain, but I think I read the quoted post differently. As to say "if the person cannot learn the skill in two hours they are never going to learn it.
In and of itself that can be true. We have likely all seen these people in class and yes, it can be frustrating from a instructor standpoint. Yes, this could be a teacher OR student issue.
What is also true and much more common is when a group of people learn a skill and do not do it enough to commit it to memory or develop muscle memory. If they did not do the skill at all for 6 months and then all came back together to practice, they would all do the skill at different levels. Either not remembering it at all (me) or doing it poorly upon first use.
Ala, a self defense 'class/demonstration'.
This idea that Anyone can teach head movement skill (or anything else) that they will be remembered 6 months later in a very stressful, fast paced encounter is the thing that is not just 'kind of silly' , It is down right dangerous and has brought the conversation full circle. What was being made fun of is now being supported.