To answer your question, Apple invited a handful of journalists to observe its testing facilities and prove that yes, the company crushed, pushed, twisted and pounded on 15,000 iPhone 6 and 6 Plus units each before unleashing them to the public.
According to the Wall Street Journal, Recode and The Verge, each phone goes through five different tests, including one that places pressure on the center of the phone in varying degrees, using weights reaching 25 kilos. Next, weights are again pressed in the middle of the phone's display hundreds of times, followed by a test that twists the device around 8,000 times while a machine is clamping both ends.
Apple also tests the phones in real-life situations, such as placing them in the back pockets of tight jeans (Samsung has its own version of the test) so they can be sat on in different angles repeatedly.
Finally, the company sends hundreds of prototypes home with select employees in order to see how they'd perform in the real world.
This isn't the first time Apple has opened its testing facilities to journalists in response to problems: back in 2010, the company also invited a handful of people to its wireless testing labs after the iPhone 4's antenna issues came to light.