Jokes aside, being part of the seed is all they care about, even if you’re just leeching? The wording in those letters sounds like they take issue in you uploading and sharing that file back.
But I’ve checked, you’re right. In their communication they don’t provide anything that proves that you uploaded a file or that you’ve done that for a set amount of time, they just use your IP address and time as proof.
So, to check if I got it right: “Never upload” is simply not possible because (1) the process of downloading something always includes uploading some information about the download and (2) copyright trolls don’t care how much you upload, what parts you’ve uploaded, what information you’ve uploaded or how long, they just care that you’re participating in the process of downloading a file, and that always includes uploading some information. They simply persecute torrenters because unlike with streaming websites, with a torrent they can easily monitor in real-time which IP is connected.