Just my two cents here: I can see some logic why document updates should not/are not rolling up in the Projects tab. When you have multiple sets of documents eg XYZA, XYZB, XYZC on the project, and you make an update on document XYZA and said, "we're not making this anymore" - and have this comment appear on the project updates tab, this can lead to confusion as to which document is being talked about or what's being referred to.
They can probably format the Projects Updates tab to show that it's a documents updates, reference the specific document but mainly what we've done is explain the reasoning above to our users (mind shift).