The article explains that. Yamaha manufactured a special trombone made of brass and other non-ferrous materials. The mouthpiece was made of plastic. The critial issue in an MRI is not metal, but any metal or alloy with iron in it. Other metals are not impacted by the magnets. good question though.
That's not true. Even a non-magnetic metallic object conducts electricity, and an electrical conductor experiences a force when it moves through a magnetic field.
If you take a piece of copper and push it between the pole pieces of a high-power magnet, you will feel a resistive force, as though you were pushing against an invisible viscous medium; this effect is described by Lenz's law.
Also, there is an all-plastic trombone called a "pBone," made by Warwick Music Ltd. in the UK.