Downloading is legal in Switzerland. For a nice authoritative source, here's the Swiss Federal Institute for Intellectual Property (part of the Department of Justice and Police)[0]: "Downloading copyright-protected works for private use is permitted in Switzerland (Art. 19 CopA)."
The mentioned legal basis is [1].
Note that this applies strictly to downloading though. Participating in a torrent swarm (where uploading is also happening) is not permitted. That's where the technicality you mention comes in: until recently, it was illegal to monitor internet users for copyright enforcement purposes, which meant it was illegal to monitor a torrent swarm, which meant you could somewhat safely seed torrents, despite it being illegal.
The mentioned legal basis is [1].
Note that this applies strictly to downloading though. Participating in a torrent swarm (where uploading is also happening) is not permitted. That's where the technicality you mention comes in: until recently, it was illegal to monitor internet users for copyright enforcement purposes, which meant it was illegal to monitor a torrent swarm, which meant you could somewhat safely seed torrents, despite it being illegal.
That loophole was recently removed however.
[0]: https://www.ige.ch/en/intellectual-property/counterfeiting-a...
[1]: https://www.fedlex.admin.ch/eli/cc/1993/1798_1798_1798/en#ar...