No, that is not the right interpretation. The rule is complicated, and 20 ppm is easy to grab onto. It is only part of the picture. There may be trace gluten from unavoidable contamination, but in no case may the amount exceed 20 ppm. To consistently achieve that, manufacturers must aim far lower due to test and batch variances. Remember, 20 is an upper limit. Zero is less than 20.
So it sounds like "gluten free" means "there's some gluten, but a minuscule amount."
The actual rule as published in the Federal Register is here. It is 95 pages long.