It is SEO spam, and TripleByte does seem to use a lot of "growth hacks" or something, because I see their ads on pretty much any platform that I see ads for (like on mobile apps, where I don't have any easy-to-use ad blockers), and they're mentioned on HN at a surprisingly high rate.
But (I think?) I've seen people here genuinely recommend them, and I also don't think YC would work with and promote a company they didn't like or trust. Companies' marketing and sales teams can sometimes have a very different vibe and sense of ethics compared to the rest of the company.
They do advertise very aggressively, to the point where I think they risk damaging their reputation. But I guess they've done the analysis and decided it's worth it.
I went through their process and got lots of practice interviewing. Even though I didn't take any of the offers I got through them I was able to use one to negotiate a much higher salary at the job I did take. Their recruiters and interviewers treated me respectfully and professionally.
So I'd recommend it, especially if you're very good and/or smart but not an attractive candidate by conventional standards. If you have 5 years at Microsoft or something then you probably don't need them, since you can interview wherever you want at that point.
But (I think?) I've seen people here genuinely recommend them, and I also don't think YC would work with and promote a company they didn't like or trust. Companies' marketing and sales teams can sometimes have a very different vibe and sense of ethics compared to the rest of the company.