Fair enough, not all mask types have equal filtration efficiencies, that's certainly not an argument I was making. Looks like masks with multiple layers of differing fabrics is key and something that can be made at home with a needle and thread:
https://pubs.acs.org/doi/10.1021/acsnano.0c03252
Unfortunately, as you noted, they're not very clear on breath collection "Exhaled breath particles were captured and differentiated into two size fractions" seems to be the extent of their description.
I can't really argue that you should or should not be surprised but like a lot of people who've read and heard various arguments that masks don't do anything, I was surprised. Doesn't it stand to reason that blocking some virus at the source would reduce the probability of transmission? I think the second article I linked shares some useful anecdotal evidence that a mask can prevent transmission. There's also this story of two infected hair stylists that failed to infect any patrons:
Hairstylists with COVID-19 didn't infect any of their 140 clients. Face masks may be why. | Live Science
It's not a controlled experiment but it's not nothing either.