It's a very small percentage yes, but I'd suggest it's fairly representative of how tech companies develop, as they generally don't exist in a bubble and valley culture always tends to be pretty homogenous. The major reason to use macs is to develop for iOS which is pretty awesome. I'm pretty sure both the companies I mentioned are fairly serious about developing their own software though.
The reason macbooks were so popular for laptop choice is due to the Chinese hacks some years ago rather than some inherent advantage. Which is why they are now dwindling. (I wasn't there at the time, but everybody at google had to hand in their laptop in 2010 or so and receive a macbook instead due to a security mandate.)
I'm not suggesting they aren't decent, (I'm typing on a macbook currently) but to say they are used for the
vast majority of development is either disingenuous or ignorant. My brother starts at Apple in 8 months (contract signed, but current company won't release him from contract early) , so I can ask him then what they all use if you'd like to revisit the discussion