They explicitly used it to “download ‘Linux isos’”, indeed!

But yeah, I don’t think Canonical would use open.demonii.si as a tracker for their torrents.

For the purposes of a criminal case, yes, they are intentionally stupid... it's up to the prosecution to prove "beyond a reasonable doubt" that something they did was illegal and that they knew it and chose to do it anyway. What a judge/jury thinks about this person, or "linux ISOs", is irrelevant... their job is only to interpret the information given to them.

There is no reasonable doubt that the domain used is for piracy and the defendant knew as much.

"Beyond a reasonable doubt" doesn't mean you can just say "no that's not true" about anything and have it not count. It's beyond a reasonable doubt, not beyond any doubt. It's not reasonable that this tracker address was gotten from a Linux ISO. Perhaps the defendant could claim they got it from a list of trackers, but they already admitted they didn't, so that's not reasonable either.