Unfortunately I think you’re right. I tested it by trying to load the Lemmy post up on a couple of single-user Mastodon instances, and they failed to load it.

It looks like only Mastodon and PeerTube and maybe a couple others list replies alongside the post. It would be nice if more did because I’m relying on the feature for this project of mine: https://github.com/BentonEdmondson/servitor

But in the use case you give, how could you view the replies below the liked comment? If the replies key is absent you can’t. So you can only see those for Mastodon/PeerTube in that case, not Lemmy/Pleroma.

I understand exactly what you're saying; you can get the parents but not the replies in that situation.

I'm just saying that in your situation (e.g. someone I follow on Mastodon likes a Lemmy post) it would be convenient for me as a user to be able to view the replies to that Lemmy post, but I cannot necessarily do that because Lemmy does not include replies.