It could be a rule your instructor made up, and there is no harm in that. In fact, it is a necessity.
As I mention in my article, there is no really reliable information on ascents to altitude like that. Before I wrote my article, I consulted with several world-renowned experts on decompression theory and asked them to help me write it. They quite rightly and quite politely refused. There is no solid research on it, and they did not want to have their names associated with anything that looks like advice and is not founded on solid science. I decided to go ahead anyway, and my article is carefully worded to say that I am only providing access to resources that will help you make up your own mind and not in any way telling you what to do.
I regularly have to ascend to altitude after diving, often after a deep decompression dive. In order to do this, I have had to come up with my own personal set of rules based upon my own interpretation of those resources, and I have had to talk to my students about it. I tell them what I do, but I also tell them that what they do is their decision. Your instructor had to do the same thing, and perhaps the 6 hour rule was his personal decision. It could also be him repeating what he was told by someone else who also had to make such a decision.