f-droid's de facto complete lack of any sort of malware is literally more important than any theoretical concern imo
is it possible to sneak malware in if you really tried? yes
is there malware? no absolutely not
you go on google play and download a flashlight app and the top 20 results are all malware lol
sure f-droid has some pretty shit apps but none of them will infect your phone, and to be fair f-droid also has a handful of extremely high quality apps that you would have trouble finding otherwise
so idk, for the average user f-droid is just de facto higher quality apps, and confers more security just by being a little more tightly controlled by forcing apps to be open source (malware people will typically not want to make their shit open source. that's usually how it works)