It's a meaningful question that the stat answers, that has nothing to do with its r2 value.
The point is that if the question is "What's one single stat that will tell me which teams will play the best defense moving forward?", all stats will let you down. Also, people will let you down.
But in a fluid game with many variables that change over time, to have one that tells even 19% of the story is actually substantial. It's not a single stat that tells all. But if you can say that "Good defense can be attributed 19% to shot suppression", then that's a big piece of the pie you're dealing with. It's not the whole thing -- there's a long tail of factors. But it's not trash or useless.
It all comes down to the question being asked of the stat.