It seems quite clear to me that Dark Tangent is attempting to stop something bad from happening at DEF CON. He doesn't want a fight or a brawl breaking out, or

It seems quite clear to me that Dark Tangent is attempting to stop something bad from happening at DEF CON. He doesn't want a fight or a brawl breaking out, or implied threats or negative community interaction. His advice isn't so much a 'no feds allowed' sign, but more of an open warning that coming as a Fed might cause undesired tension and circumstances that weren't previously present in such high volumes. Calling the situation ironic because DT is a fed is unrelated - he may work for the government, but he isn't in a position that has a conflict of interest with a hacker conference (read: he doesn't work for the NSA). If you say "Dark Tangent" to a person in the security community, people recognize him as the creator of a hacker conference; not a government employee.