We'd need for lemmy to turn into a single link aggregator, into a multiple, for that to work.
Lemmy
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I think you misunderstand me. There is only one link. It is a link to a mastodon post. That post has 4 images. Reddit is able to take a imgur gallery (which is a single link) and preview all the images in that gallery. I don't know exactly how it does this but I imagine it's probably doing some kind of web crawling when it detects that it is a imgur gallery and then crawls the separate images that are found on the page that the gallery link leads to.
That's because reddit is their own image host, and built in multi picture functionality. We use pictrs as our picture host, and they'd rather not create albums. Either way lemmy-native-albums wouldn't be good for federation anyway.
But this works when it's hosted on imgur too. Or maybe that's just an RES feature I forgot wasn't native.
Also I use the third party app client Boost which does this too. But again it's not the actual reddit site, it's probably doing some extra work.
imgur is a proprietary platform that also has their own unique way of doing albums. Boost is using the special reddit album feature.