This isn't insider trading for several reasons. They aren't allowed to use non-public information for personal gain, and they neither gained nor were they privy to any new laws or policies that would have caused stocks to rise or shrink, and the evidence is that they didn't even sell for another 3 weeks after this "secret meeting".
Even though there was no gain, it would probably still qualify if they were informed of some new law or bill that the rest of us didn't know about that would cause the stock market to move. AFAIK, the public already knew about coronavirus before the briefing, they didn't set any federal policies at the meeting, otherwise they wouldn't have waited 3 weeks to sell, and most of the measures have been at the state and local level, and senators work for the federal government.