It's in reference to the Sakuracon convention that Penny-Aracde had a booth at. There are apparently a lot of cosplayers and some were wearing TF2 outfits this year. Cosplayers who have swords or any weapons (which I'm sure a lot do) have to agree to a peacebonding and wear a tag on their weapon saying that they agree not to use it.
Yeah, it was really cool meeting "Gabe" and "Tycho" at Comic-Con in SD. That was back in high school, and I haven't made it in like 6 years...I was really nervous seeing Lou Ferigno (The Hulk) and just stared at his colossal-nous.