You're conflating some things here. Without unpicking it all, the main thing I'd say is that you've only demonstrated that amazing hiring is not sufficient for competitive advantage. You have not demonstrated that it's not necessary.
I'd say there's a big difference between a company finding engineers and engineers throwing themselves at a company. The latter is what often occurs with BigCos. They have a big pool of candidates throwing themselves at the company, and the company gets to pick whomever they like from the pool. How much credit to due to the company for picking? Whereas I think you'd give a company more credit for going out and specifically recruiting/poaching one person they want, even if that person wasn't applying for a job with them.
Lots. Firstly for becoming a place people want to work. Secondly for sifting through all of those people.
Only if you view hiring as the competitive advantage of Google/Microsoft/Apple. But everyone cargo cults the hiring practices of Google/Microsoft/Apple without becoming Google/Microsoft/Apple, which shows that it's not actually some magical unique competitive advantage.
We can get into a discussion/argument about what is the competitive advantage of Google/Microsoft/Apple (which collectively own near 100% of the market for mobile OS, desktop OS, web browser, search, etc.), but that's kind of a tangent IMO.
I will say that Google/Microsoft/Apple have a competitive advantage specifically in hiring (that is, getting many of the top candidates to apply) because of their prestige (a product of the aforementioned market dominance) and vast wealth. The latter in particular allows them to offer more compensation, especially stock compensation, than other companies.
It's important to mention though that the formalized hiring practices of these companies are largely ex post facto and could only be put in place after they were already successful. It's unlikely they hired exactly in the same way during the early years. Steve Jobs was introduced to Steve Wozniack by a friend in high school! Nobody can plausibly claim that the success of the Apple II was due to "coding interviews". Larry Page and Sergey Brin met in college, where they were already working on the ideas that led to Google. Hiring doesn't just turn into success, that's not a reliable formula.
You can set the bar as high as you like, but you won't be able to hire anyone if candidates just walk away from your obstacle course. Talented people put themselves through a lot of crap to work at BigCos because there's a big payoff working for a BigCo. That's what tends to make the BigCo hiring pool better than for other companies, not the hiring bar itself. You can afford to be extremely picky if you have a wealth of options to pick from.