They are only talking about slow, conservative, stop-the-world collectors there.
They have to scan the full heap. You shouldn't compare apples with oranges.
I find the methodology of the research to be solid, the collectors to be representative of the different approaches in the field and the conclusions to be consistent with the gut feeling from experience:
"With only three times as much memory, the collector runs on average 17% slower than explicit memory management. However, with only twice as much memory, garbage collection degrades performance by nearly 70%."
"With only three times as much memory, the collector runs on average 17% slower than explicit memory management. However, with only twice as much memory, garbage collection degrades performance by nearly 70%."