A post that only contains a quote from another post and nothing else doesn’t add much to the conversation. Post quotes are stripped out when calculating the post length. So, the post length is effectively 0 if that’s all the post contains. That’s why you see that error. It’s been that way for a long time.
When you quote a post in Discourse, you get this in the composer.
[quote="user, post:1, topic:558"]
some quote...
[/quote]
All of those characters are not counted.
See here
and here
This is very different from a generic quote like this
> Most people do not need more space, they need less stuff. ― Michelle Ogundehin
which renders
Most people do not need more space, they need less stuff. ― Michelle Ogundehin
since I’m presumably adding to the conversation rather than reposting something from another post without adding to it.