I am a big Apple user, in many senses really a fan. But these scare tactics are just off. First of all, the safety promises of the Apple store are way worse than Apple makes them. There is tons of dubious apps around which rob users quickly, just barely within the guide lines. Often enough, bad apps slip through the nets. On the other side, many absolutely legit apps get blocked due to bad review or overzealous restrictions.
Would it be a bad idea to install apps randomly from everywhere? Sure! Would it be a security risk to install well known open source software from a reputably curated alternate store? Most likely not! Also, the system should have more robust implementation which would prevent malicious apps of doing damage until the user explicitly grants entitlements.
And I would be much less interested in installing apps from different sources, if the content of the app store were just guided by safety concerns. But a ton of stuff is blocked just becaue Apple wants to block it. That is, where the demand for alternatives comes in. And lets not even talk about adult content. It would be great for the safety of the users, if Apple would allow such content within the protections of the App Store. But no, even though you have solid age information about your users, it is banned and people are driven to sources outside of the App Store safety and outside of secure payments.