Mastodon and lemmy handle this in slightly different ways. Mastodon (according to the link) replicates media on every instance while lemmy (mostly) only replicates thumbnails. That means a popular post doesn’t cause load for one server on mastodon but does on lemmy. But Mastodon has a higher aggregate cost due to all the replicated data, which is what the linked proposal solves by making it sublinear.
If the torrent is instance to instance I don’t see any real benefit (and instance to client is infeasible). On Mastodon side you still have data duplication driving storage costs and bandwidth usage regardless of whether it’s delivered via direct http or torrent. On the lemmy side it wouldn’t gain much (asymmetric load is based on subscription count and so not very bursty) but would add a lot of non-determinism and complexity to the already fragile federation process.
Conventional solutions like cache/CDN/Object Storage or switching to a shared hosting solution (decoupled from instances like your link proposes) seems like a more feasible way to address things.
Data size and user expectations is the main difference. It’s possible but there’d be a lot of latency and overhead for just scrolling down a page with a bunch of images. Maybe there’s fancy stuff you could do by batching images together and reusing connection pools but it feels sisyphean.