Clarification: at 30k ft?
Yes, the comment is intended for
roughly that altitude. 99% a good ballpark figure given that you can usually quite easily see contrails (and clouds) over 100 miles away. So the area of a 10 mile radius circle (100 square mile) is just 1% the area of a 100 mile radius circle (10,000 square miles).
The actual amount varies with exactly how you measure it. For example, you can actually see around 200 miles if you are looking at things at typical contrail altitude, so you could say that 99.9975% of the sky is over 10 miles away. However it's quite hard to see things over 100 miles. I commonly see planes leaving contrails off the coast of San Francisco, those are about 115 miles away.
Perhaps a better way of putting it would be "Over 99% of contrails that you see are over 10 miles away"