Most people who say that do so for dogmatic reasons, not because they arrived at this conclusion after careful analysis. It's the political point of small government.
These are the same people who will probably be first in line shouting for government intervention when their drinking water is full of chemical waste.
You can try to reason with folks like that but you probably won't change their mind. Just try not to shout at them.
Some of the technical info flew right over my head in the first article. What I took from the piece is that he has valid points so far as I can see and understand it. I would say nevertheless the author was a bit biased as well. And it's 3 years old. It may still be accurate, IDK.
I use F-Droid and have been for a while and I'm not aware of any issues this could've caused me. But I'm also not using it for essential systems. Not for browsers, VPN, etc. I have downloaded games, a couple of notes apps, that sort of thing. I would never recommend you get all your apps from there. It's an addition to Google or your usual poison.
Security experts will never be happy; that's their job. The author is also talking about your threat model. Are you okay with certain risks? The truth is also that somebody could screw you over on Google Play. It may be less likely comparatively but not impossible. So you try to jump from rock to rock hoping no alligator catches you. So far no alligator got me.