I have a state of the art Macbook too, and a good monitor, but I still find light grey text hard to read. Because I have old eyes with suboptimal eyesight.
Anyway, it would be reasonable if designers tested their designs with other people on other hardware, instead of thinking "looks fine for me" … at least that's what I learned about design, user interfaces, etc ;-0
I think much more reasonable is the assumption that many designers have Macs with good monitors and there you can still read it well.
Whereas most people have old or poorly color calibrated monitors where you only get a grey goo.