The post has since been updated, but I'll just copy it here: This isn't about actually coming in on a Sunday (most people don't) — it's to determine if this is someone who is so impressive and so nice that others actively want to be around them.
Also, as mentioned in the post, usually just the interviewers attend the meeting to make a hiring decision. In the case that someone didn't interview a candidate, but knows him or her in some other way, we want to make sure this person can also join in, if wanted.
The post has since been updated, but I'll just copy it here: This isn't about actually coming in on a Sunday (most people don't) — it's to determine if this is someone who is so impressive and so nice that others actively want to be around them.
Also, as mentioned in the post, usually just the interviewers attend the meeting to make a hiring decision. In the case that someone didn't interview a candidate, but knows him or her in some other way, we want to make sure this person can also join in, if wanted.