Maybe so, but plain aluminium won't have the proper pattern for the quadrature encoder to give an accurate result.
I see no pattern on the original slider - OK, maybe it's on the backside of the aluminum strip of the slider.
Curious
The typical digital vernier has a plastic cover over the patterned 'pcb'
Is the aluminium strip actually aluminium or just aluminium coloured (like paint)?
A typical read-head will see almost any change of capacitance as movement, so I'm no too surprised a plain metal strip will cause the digits to change, but I would be surprised if they have found a way to 'read' the surface capacitively in a similar way an optical mouse can with reflected light.
Bill