They both also did that - Goerner with weights, Sass with isometrics and odd objects.Or they could have just used a progressive resistance routine to get stronger generally and practiced the sport?
You are getting basically to the point that barbell training is useless for sports. It couldnt be farther from the truth.
No, I am not saying barbells are useless for sports at all. I think barbells and dumbbells are among the simplest ways to incorporate general resistance training for most sport, and have the advantage their use can be coached by anyone with barbell experience, rather than someone who is intrivately familiar with each sport. However, I have also been arguing that there are different alternatives, especially if the sport itself includes resistance to a high degree, whether it's your own weight, the weight of an opponent or an odd implement, the muscular force of the opponent or something that mimics that, like a band. All of these have advantages and disadvantages, and need to be incorporated in the total training program to yield optimal results. It's as simple as that.
If you want to take up Basque stone lifting, you could say "I'll get my deadlift to 700 and my squat to 600 first, then it will be a piece of cake". Or you could just start lifting the stones as a youth, like they do. By the time you get to your target numbers, the Basque kids will already be lifting the 220 kg stone for reps, even though their squat and deadlift won't be nearly as good as yours. You could also lift stones and do squats and deadlifts on the side, though your stone lifting technique probably won't get as good if you only lift the stones 2x vs their 4x. Or you could be content with your current 550 lbs deadlift and 450 squat and take up stone lifting full time. On the other hand, if said Basque kids do zero GPP on the side, their careers will likely be cut short by overuse injuries etc.