I'm not sure what proof people have that some devices are "impossible to remotely hack".
It is easy to prove plausibility.
There are a host of possibilities to gain remote access.
Most importantly, if remote control is possible on a jail broken device, then all that remains is an attacker gaining access to remote jail break.
Proofs:
1.How to Install VNC Server on Jailbroken iOS Device
via LifeHacker
2. Man in the Middle Attack Demonstrated by McAfee
via SecurityWeek - attacker gains access, silently jailbreaks, then installs VNC server.
While the OP may have had a bad pop-up, and although the McAfee exploit report is old, here are a few ways to protect yourself:
1. lock your phone - prevent people from physically tampering with your device
2. avoid public (rogue) WIFI hotspots, even better, use a VPN over WIFI or stay on 4G
3. be mindful of pop-ups posing as Apple. If you get an Apple pop-up to call them, take it to Genius Bar instead.
4. keep iOS updated. while this is difficult for many, nowadays it is even more important to patch exploits.
5. close browser vulnerabilities, such as the solution in this thread
6. consider getting a security app, be it antivirus/antimalware - let it be someone else's job to keep an eye out for you
Hope this helps anyone concerned or affected.