Phabricator is a collection of software development applications similar to the Atlassian suite (code review, wiki, repositories, bug tracking, etc).
All the content is user-generated, so we can’t generate a useful synthetic description in the general case: we have no idea what a particular wiki page is for, or why a user didn’t add a description to a bug, or why a commit message is blank, why a cluster topology record exists, or why a blog post is just a picture of a cat. In many cases, these resources are still useful and relevant in context.
I imagine users posting or reading links to Phabricator are likely have a good understanding of that context. It’s unlikely that you’d go to discourse.digimon-enthusiasts.net and post a link to a Phabricator install, and more likely that you’d go to discourse.seriouscompany.com and post a link to, e.g., a bug in that company’s bug tracker or a page in their software documentation which might be hosted on Phabricator, which most users reading the discussion would understand.
In fact, OneBox already generates a similar box for GitHub issues with no description:
https://github.com/epriestley/poems/issues/4
And for commits with no message:
https://github.com/epriestley/poems/commit/da741f11202f505ead4514591d837b729bb8ce63
So maybe this is a bug with many existing OneBox integrations instead, where the implementation fails users when resources have no natural description?