The 'contradiction' stems from individual opinion.

Since security, like nirvana, is a state that is not reachable with any combination of operating system and application on earth, individuals pick their security comfort zone individually. Hence the apparent contradiction.

People in the Linux department mostly think that viruses is something that happens to Windows only and applying antiviral measures to Linux itself would be pointless. Given the net effectiveness of viral protection that happens at the Windows camp, some even forward the opinion that the entire business model is shoddy, scare-ware or a scam in itself.

Please read the many articles you refer to and form your own opinion.

No, neither ubuntu nor xubuntu come with anti-virus or a firewall pre-installed. The makers believe that the system is as secure as it gets.