Looks like a basic endurance training. With that said, from personal experience, I've seen the most gain in stamina from trying to get faster. Increased finger control and coordination always made lower bpms easier and easier to maintain and what you consider low bpm also changes with that.
While something like this can surely be an isolated "workout" for an experienced player, I would think that it's better for everyone that isn't to play in the actual client to get feedback on their timing, cause accuracy is way more important than simply having "resistance/endurance". Playing long streams to simualate the same thing would already be a better alternative, as long as you actually try to acc and not just mash.
With all of that said, I'd consider it to be a much better use of anyones time to work on their sightreading/acc as their first priority and playing burst maps occasionally to get faster than to play long stream maps to practice endurance. It rewards you with actually important skills and you get stamina from it as a side effect too, alongside being able to play higher bpms, you just get way more in total like this.