Hmmm. So the app is making a request to the website by passively scrolling through the feed? That seems pretty bad for security/privacy reasons. I'd much rather see the missing thumbnail scraping fixed in Lemmy. And then it will work in all apps, and not have privacy issues.
I'm not sure then, that would be a question for those app developers. The thumbnail URL doesn't exist in Lemmy's API. You can see the same behavior in lemmy-ui.
The post you linked to doesn't have a thumbnail returned from the lemmy.world API. Were you viewing that post through another instance of Lemmy that happened to pick up a thumbnail?
Any service can implement this today, with activitypub. Being an enhancement proposal is just an attempt to standardize extensions to ActivityPub, lots of the time that services have already implemented.
Mostly because I don't want to spend my free time supporting outdated lemmy versions.