Jailbreaks exploit vulnerabilities. There should be a sanctioned way to put an iOS device into an “untrusted” mode that doesn’t rely on vulnerabilities. That doesn’t harm your security at all.

That aside, the vulnerabilities (made by Apple) are what make your iOS device insecure— not the jailbreakers who use them.

The vulnerabilities in Apple’s code that jailbreaks exploit make your iOS device less secure.