They know what torrents people download by IP.

anyone can figure that out: https://iknowwhatyoudownload.com/

Associating IPs with social media accounts is a step towards identifying people so they can threaten them and force them into settlements.

It's a numbers game for the lawyers. They want as much data as they can get to identify the largest number of people so they can demand an out-of-court settlement.

The "DMing pictures" part is just an example of how they could gather that kind of data from a social network like Lemmy that can't be so easily subpoenaed, and allows image hotlinking. I don't have any evidence that they are doing this (yet), I just know that it would work.