I don't like the grandma analogy, some old people are really smart with computers. But in general, if you're in that situation with someone where you're worried that they'll get scammed if you leave them alone for a second, at some point you need to ask whether they should have access to an app store at all.
How many apps per week does your grandma need to install? It would be better if you set her up a phone with the apps she wants, and then disabled all app installation.
Because if someone is in that position, the unfortunate reality is that the official app store isn't safe either. The article gets into this in more detail, but there's enough malware on the official Apple Store to make it dangerous to randomly install apps. There are apps where their whole design is to set you up with big subscriptions in the background that you don't notice, there are malware apps that slip through Apple's review process, there are phishing apps.
Really, you should install the apps your grandma needs, and if she needs more later, then she can ask you about them (or someone else who's an expert). I think people look at the Apple Store as if it's perfectly safe and that opening it up would suddenly let in malware for the first time. But while the Apple Store might have comparatively less malware than Android, that's not the same thing as being perfectly safe, and it doesn't mean you can let a young kid or a naive adult go wild on it and install whatever they want. That's a recipe for disaster.
I've set people up on Linux and had zero support calls or malware problems with them, not because Linux has good security or perfectly curated software sources, and not because there aren't dangerous ways to get malware on a command line, but because they don't open the command line in the first place. Some people are safer and thrive in a computing environment that's set up to do the things they want and that doesn't change after that point -- but I don't think that has much of anything to do with alternative app stores, that's really a question of whether app stores should be allowed at all for those people.