You have no evidence of that not happening. It could be corruption after the fact or failure during replication.

The armchair wolves already smell blood and are assigning blame before a postmortem has even begun.

You're right. A headline of "Dell's website is serving up unsigned updates" would be correct. But to garner more clicks and hype that's not how they've worded their tweet, instead it's worded to make it sound like Dell are doing this on purpose.