Maybe not perfect English, but I'd expect a significantly higher level of English than that quote demonstrates. Add in a stock photo and why would anyone think that that is real?
Being a good writer generally doesn't matter in high school. Most classes don't require much writing and for those that do, they just require students to convey the right number of facts and avoid most grammar mistakes.
Kids also engage in far more code-switching than adults, I think. When asked for a recommendation, It's possible that this particular kid just dashed off a quick email rather than really sit down to write something engaging.
But seriously, if I were doing a Y Combinator application, I would make sure that my testimonials were spot on. I would do so because one of Y Combinators' most enduring bits of advice is 'build something people want.'
I'd even equate that to grammatical errors in a website. You're competing for an interview against thousands of the best minds around. Proofread your copy!
Rather, I'm saying that when you apply to Y Combinator, you are competing against the best entrepreneurs around for a very limited number of seats. Since Y Combinator is so competitive, your chances of getting in would be better if you started working with a good writer or editor.