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Second possibility is that it was not so much about military secrets, but rather social class, as horsemanship was a generally required skill for nobility and these skills were taught as part of a "gentleman's" education. This was up until the time that European nobility began the intermarriage process with new moneyed classes, the related economic phenomena of companies such as East India, and the related military matters of colonizing (something new for Europe) far flung lands. Those changes to social order and military requirements likely gave us the impetus for "commoners" on horses; i.e. scaling the officer class ranks beyond aristocracy. Your Dom Duarte's timeline fits well with that speculation.

https://wiktenauer.com/wiki/Category:Mounted_Fencing

A classic one is from Von Danzig: https://wiktenauer.com/wiki/Pseudo-Peter_von_Danzig#Mounted_...

People have been researching this since the early 2010s: https://youtu.be/AYxZcZE0Yyk (HEMAC Dijon 2013)

But to answer your question, many martial artists train with weapons. It's fairly easy to use a dummy knife to simulate real combat without harming anyone. And many fighters throughout history would have trained with weapons to simulate real combat. I cannot speak for these knife techniques in particular because that was never my specialty, but there are plenty of legitimate techniques out there. In terms of what they are teaching in self defense classes, I find that a very mixed bag. Some focus strictly on tricks that seem cool to the untrained, but are hard to pull off in real life without a great deal of practice, while some instructors take a more realistic approach (be aware, have a plan, use a minimum of physical technique to extricate yourself as quickly as possible)

How likely are you to use them in an actual confrontation though? Probably not likely, you would have to be put in a situation where other forms of neutralizing the situation are not possible (this includes running the fuck away); and at that point, primary importance is having a very limited set of general techniques that can be applied in multiple situation with the weapon so that through practice and training it's more of an autonomic function of your body (you don't want the lag of having to 'think' about which technique to use); secondary importance is experience with these situations - i.e. one of the marital arts groups I worked with would have us go to, and shoot at, a firing range so that we were used to the sounds that a gun would make at close range so that we were used to it in the unenviable case of being in that situation.

Generally, disarm techniques are going to be extremely similar, but with any sharp object it's going to be - expect to get cut, deflect with non-vital parts of your body (such as the outsides of your arms, not the supple insides of your wrists), and neutralize the weapon as quickly and safely as possible. Some martial arts systems even have scarf techniques for doing disarms with soft objects from a distance (it's super cool seeing someone do a gun disarm with a cloth/scarf).

If a technique shows up in various treaties from multiple countries, and cultures, it is quite likely legit in its context. Also funny how similar, equal even, HEMA wrestling and Japanese Jiu-Jitsu is. Not surprising, both thing are a solution to the problem going close with someone with a bladed weapon.

Martial arts is about raising the probability of your opponent dying before you. So if two people had knives, the person trained with knife fighting would more than likely be better off than the person who didn’t have raining.

... but what’s also been taught in different schools of Kung Fu, is if they have a knife and you don’t have anything - run!