I think that would go for most learning methods. When you don't practice a skill you're always going to get worse with it over time, especially if it is a language.

According to some guy on Youtube, that's less of a learning method thing and more of a getting over a basic threshold of competency thing. I forget exactly which level he said it was, but the claim was that if you reach at least B2(?) you won't forget it anymore.

I think this heavily depends on your learning type. For some it may work for others not. What is important that it actually helps some people and these people have no foss alternative around.