The left side has a navigation to different translations of the page. All but the English version link to the German page as well.
I'm guessing it was once part of the build pipeline but has since been abandoned.
So yeah, it is an official source. It's just outdated
Unfortunately, 7-zip barely has any security involved. No digital signatures, no ASLR, no NX bit, no stack canaries, no nothing.
Hopefully these security concerns wake up Ivor. Its not the 90s anymore: developers have to participate to get a proper security posture. That's why Windows tried so hard to get everyone to use sandboxed Win10 Apps / Metro Sandbox by default, because these problems require the developers to care about security.
`whois 7-zip.de` resolves to a private person in Germany. This does not look official to me. More like a crowdsourced effort of providing translated websites with a dangerous effect in case of security vulnerabilities.
The versions provided are (as of 2018-05-04T10:20:00Z): en 18.05, de 16.04, zh 16.04/18.05, eo 18.01, fr 18.01, ja 18.05, pt 18.01, es 18.01, th 18.05, vi 18.01