No I've heard this too. Look at pictures of him next to Demarcus Cousins. Durant is a legit 7'.
Cousins is not even 6'11 in shoes...
edit: Either way, this argument is largely semantic. I'll grant you that Durant is likely closer to 6'11 than to 6'9, but his inclusion in this example is odd either way.
Tall guys have durability issues not because they're tall, but because they're generally heavier and jumping up and down for a living when you're ~300 lbs. can do a number on your lower body and your ankles and knees in specific.
For Durant, this isn't really an issue for obvious reasons. Although for really tall guys being rail-thin isn't too good either -- you're more likely to get injured when there isn't enough muscle to support the joints.