First: self-reporting isn't predictably reliable. There's something called social desirability bias where people will answer in a way they perceive will be well-received. They can overstate the good or understate the bad. The people responding to the survey already have Covid, and it's quite possible that they want to believe they "always" wore a mask and therefore aren't responsible for getting Covid. It's not necessarily a character defect to want to believe the best about oneself, it's just human nature. But it might not be accurate.
Second: 51% of cases were acquired from family members. How many of us wear masks in our family bubble?
Third: I can't remember the percentage but one of the places the patients had more commonly been in the previous 14 days was restaurants. People can't eat and wear masks and at least where I live you're not required to wear them once you're seated at a table. But for those eating indoors, circulated air can pose a greater risk of exposure to the virus.
I'm not saying masks will always work, but on the flip side they won't always fail and they aren't going to infect you (if you're wearing them and handling them the way you should) more than if you hadn't worn them at all. So using this as a reason not to wear a mask? It doesn't make the case.