By the text of the article, it does not seem to have been an ILLEGAL act.
It is illegal to fly under bridge under any circumstances.
In the US, under the Federal Aviation Regulations (FAR) it’s usually illegal to fly under or even particularly near a “person, vessel, vehicle, or structure.” FAR 91.119 specifies the Minimum Safe Altitudes above and near man made objects.However, under certain conditions, it is legal to fly under a bridge here in the US.
Under that proviso I’ve flown under a particular bridge in California a number of times.
As a “seaplane” pilot it is legal to fly under a bridge if the act of flying under the bridge is required as part of a safe takeoff.