No, you do not know they’re 99%. You cannot be sure any vendor’s testing is not cherry picked for positive results, or that they haven’t done something with the batch that causes premature degradation leading to lower purity by the time you get it. This includes nexa who has made mixed statements about whether they outsource their production to who knows who in China, or are DIYing it in their garage here in the USA, probably some combination of the two but they like to lie so hard to evaluate. There is some evidence that they toy with ingredients and that has caused degradation issues, though.
Point is that post-purchase testing is an absolutely critical step, it isn’t just for funsies. Vendors’ tests are marketing tools, not harm reduction tools. Consider the incentive structure they’re working within.