Those micro-thrusters are probably piezoelectric, like the nozzles on ink jet printers.

They move a bit I guess, but there is no net motion - they transmit no force or vibration outside of the device. They flex, but have no relative (sliding) motion.

There are other parts on the satellite that can bend - will you call that a moving part as well?

Piezoelectric actuator still relies on a mechanical motion. It has to be - the idea of nitrogen nozzle is to have a storage of nitrogen under some pressure in a closed volume, and to be able to open that volume to let some gas out. That opening has to be mechanical - unless they use some exotic force fields to keep nitrogen from escaping, in which case it's hardly a (regular) nitrogen thruster anymore.

Flexing is still a motion. That, however, can be compensated with an opposite motion in another gas channel.

If other parts of satellite can bend on demand, they'd need an actuator or, alternatively, one can use Earth magnetic fields to reorient the spacecraft and let external forces affect it unevenly, thus providing bending. This, however, is outside of the scope of the article.

So, to summarize - either secondary subsystems on Gaia are quite unconventional, or the article isn't quite precise.

Conventionally a moving part is one which moves relative to another, but flexing parts are not included in the definition.

The reason for excluding them is that EVERYTHING can flex. So if you included that in the definition it would make it meaningless.