First things first. You don’t have to be as naturally talented as you’re thinking. As long as you are a sincere worker, who can focus on solving problems, that’s all they are looking for.
First of all, there is no good way to measure talent. In popular culture, we measure it with IQ. But there is also a large body of research which says that IQ is not correlated with success. But even if you don’t believe this; how do you decipher talent by looking at someone’s resume? Resumes are usually not very well written and often don’t tell the truth either.
Google doesn’t insist on only interviewing talented people is because they hire massively on scale. i.e., thousands a month, across the globe. If they insist on interviewing only people with talent, they won’t ever be able to hire and push great products out fast.
Google does need people with high IQ but not every position per se needs high IQ. People who are diligent and have the right approach are also critical to any big company’s success. In fact, that is how large companies are built. There are maybe 10% of positions which may need innate talent to be successfully producing right output, but the other 90% just requires sincerity and dedication to solving tough problems. Talent is massively overrated in such positions.
Finally and most fundamentally, they have great faith in their own interview process. The interview process is rigorous and only selects the best prepared candidates. Interview preparation for Google is a great proxy for your sincerity and dedication to solving tough problems.
The only time they have to use a loose proxy for talent, is when there are just too many applicants and very few positions to fill .e.g., freshers in satellite locations. There are limited fresher openings and if they go out accepting candidate profiles from everywhere, they will never be able to get through the interview process. So they prefer to go to top colleges, which are loose proxies for talent e.g., IITs in India. Even here, if you have a strong referral, you will get interviews regardless of whatever college you went to.