This is probably a fairly minor thing, but it seems that the WordPress plugin simply ignores the character limit on Discourse. Sure, it gives a warning if the post in question exceeds the character limit, but it still posts it in its entirety on Discourse. I expect this is not the intended behavior.
n.b., I have not tested if this breaks anything if the post is excessively long.
The reason the post still gets published is because the WP Discourse plugin sets the skip_validations parameter to true when it publishes a post to Discourse. I think the plugin needs to keep setting that parameter, but probably does need to do a check for the post’s length. WordPress posts can potentially be very long.