I'm watching Terror in Resonance, the airport episode. Sphinx goes through a lot of trouble to find the hidden bomb - even risking their own identities. Their intention is to keep everyone away from harm.
Why don't they just upload a new video literally saying "we're gonna blow up the airport"?
At this point, Sphinx has gained so much media influence that it would instantly go viral (just like with their previous videos). The airport (and anyone else) would most certainly take such video seriously and evacuate.
This way, there would be no casualties, which is what they want. The bomb would still detonate and cause several structural damages, but that would not be anything new or surprising considering Sphinx literally blew up a skyscraper in the first episode anyway.