That’s because there are bots doing that, and they were designed specifically to bring parts of reddit to lemmy, without having to directly interface with reddit’s site, app, or issues
The key is you block the bots, usually. The lemmit bot only posts to the lemmit instance (afaik), so you can also just filter/block the instance itself in that case.
Once the bots are blocked, you usually end up with no reddit reposts because there aren’t that many of the bots, but they post relatively broadly.