Oh yeah, fighters have always done 'interval' work, or wind sprints, hill running etc. Training hard and for fitness is actually quite straight-forward and athletes figured most of it out decades ago, perhaps millennia ago ! Trial and error, common sense and instinct.
I think in 'the information age' and with everything now a body of science, some people anyone who lived 50 years ago couldn't have had a clue about anything.
People might use words like 'plyometrics' and shit now, doesn't mean the method didn't exist a long time before the word was coined and some geeks wrote a book on it.