Let me translate the key section of his letter: "Section 86a of the Criminal Code is located in the third title of the first section of the Special Part of the Criminal Code. The offenses of this title criminalize acts that constitute a 'threat to the democratic constitutional state.'" The protected interests of the provision are political peace, the free democratic basic order democratic basic order, the idea of international understanding, and Germany's reputation abroad. The ban serves [...] to prevent the revival of the banned organizations or the aspirations they pursue [...]."
"Hitler.rocks" is not falling under this, since it is not a "threat to the democratic constitutional state". That's also why satire magazines like "Titanic" are allowed to publicly show Swastikas on their front page. [2]
A domain about hitler minerals [3] which does not aspire the revival of banned organizations such as the NSDAP, (there isn't any nazi glorifying content on the website), does not reflect a threat to Germany's democratic state. At least I hope so :)
[1] https://www.bundestag.de/resource/blob/869290/c8bd5f14ef172e...
I feel like the law would be especially difficult to apply in this context given that the domain names offered by cock.li are obviously picked for the sole purpose of causing offense, not to promote an unconstitutional organization.
Not complying with court orders for information disclosure is also either a crime or a serious infraction (Ordnungswidrigkeit), too.
As said, the guy got lucky he didn't get arrested like he should have been.