Security is not there to safeguard you from stupidity.

Not really, it's not much of an issue on Android. iOS will probably do the same, but on Android if you sideload an app that could be malicious, the Play Store has play protect and scans malicious apks like an anti virus. But also phones are much better sandboxed and secured then desktop, so their security against malicious software is much stronger.

While not impossible, you have to try fairly hard to fuck up your phone like this. I'd be actively impressed if your story is true (particularly as you used the plural), and if so I'd like to know what they were specifically trying to install that fucked up their phones.

It's just statistically more likely they downloaded a malicious app from the Play Store than had any chaos side loading.