AUR has no connection to the system as you describe in the above quote.
Access to AUR is a courtesy of Arch Linux - and the service may or may not be available depending on maintenance or ongoing DoS or DDoS attacks on the Arch Linux infrastructure.
pamac can build custom scripts - such as those available via AUR - even given that - custom scripts are unsupported whether you build using pamac or any other AUR-helper.
AUR is Arch User Repository and may cause issues with your Manjaro installation - especially if you are using stable branch; regarding issues, testing and unstable branch are more forgiving.