Perhaps he's more skilled at Windows than at Linux? In which case, if he were to switch to a platform that's unfamiliar, he would be more likely to make an erro

Perhaps he's more skilled at Windows than at Linux? In which case, if he were to switch to a platform that's unfamiliar, he would be more likely to make an error that would lead him to being vulnerable. I'd suggest that by using a platform that he knows how to make secure rather than one he's unfamiliar with, he is acting in the best way to ensure the security of his device.