As the paper points out that's accounted for in the analysis, because shockingly enough to most internet commentators researchers know that they need to control for confounding variables
> "researchers know that they need to control for confounding variables"
'Why ‟controlling for a variable” doesn't (usually) work' - https://dynomight.net/control/
Other things that don't work, including four other 'controlling for a variable' entries at items 39 through 43: https://dynomight.net/things/
Always insane to me how people will just read the headline of an article and point out "flaws" based on what they assume the article says