But part of the flight procedure is to actually unlock the feathers slightly later in the flight - at Mach 1.4.
The article states this is to make sure the feathers are operational before heading upward, but that seems like a really narrow margin of time to execute this test.
While speed is a factor, it isn't the only factor. The altitude at which they are unlocked matters as well. Moving at Mach 1.0 at a lower altitude could subject the plane to far greater forces than at Mach 1.4 at a much higher altitude, where the air is thinner.
Furthermore, where the force is coming from probably also plays a role. The angle of the force applied to the feathers may change when the force is not coming from a fall, but is instead the result of the engines provide force from behind the fuselage.
The article states this is to make sure the feathers are operational before heading upward, but that seems like a really narrow margin of time to execute this test.