This confused me too, but I believe they're saying "if you upload (for example) a photo to Instagram, we'll still allow you to upload it to Twitter also, but you're not allowed to link to it".
I find it strange they explicitly mention this, though. As if they might decide only completely original media is allowed on Twitter.
What it indicates to me is that they considered blocking apps that make it easy for someone to post something on multiple social media platforms at the same time.
Which is why they specifically mentioned that this is allowed, since it was something that was top of the mind of whoever made the decision.
I think you might be right. If they're refusing to allow you to post something on one network and then link to it on Twitter, that is hilariously petty.
All this is going to do is drive creators to mastodon...which means at least for the popular ones, they'll end up gladly giving mastodon a lot of extra visibility.
I find the language itself to be pretty clear, but since it would be absolutely insane* if crossposting the same content was prohibited, you think that they simply must have meant something else with that statement. Hence the confusion.
[*] To us, but apparently not to them, so let's see where this train is headed next.
I find it strange they explicitly mention this, though. As if they might decide only completely original media is allowed on Twitter.