Read it, it doesn't state up to how many significant digits that percent is accurate so it doesn't prove I'm wrong. They may have just shorten it for brevity.
The convention in science is that you display as many digits as your measurement is significant to. If it were significant to two digits, it would've been 99%; if it were significant to four digits, it would've been 99.90%.
In any case, that number's backed up by several sources:
