I'm a bit of two minds about this. On the one hand you're quite right, it's impressive work for high school students. What I don't like is how pointing that out feels like an insult, when what I really want to convey is that it's impressive but not entirely beyond ordinary high school students with an interest in mathematics.

And articles like this have been popping up for years (I think about the exact same two students even), and each time I have to decide whether to downplay the scale of their achievement so high school students don't lose hope about achieving something similar, or praise them with the qualifier for high school students because they couldn't be expected to have enough mathematical background to push the boundaries of one of the oldest and most extensively researched parts of modern mathematics.

I can't help but feel that each additional article is just further entrenching the stereotype that you're either a genius at mathematics or not, and is demotivating the students in question, because how on earth are they ever going to top this?