Opaque blocks can be indirectly powered using a repeater

I have no clue what is going on, nor how to describe this. Just replicate the circuit shown in my screenshot (yes, including that nonsensical repeater) and turn on the lever. The sticky piston will extend for no apparent reason. Any mechanism components can be activated this way, not just pistons.

Slime blocks are indeed opaque so it's normal they conduct a redstone current, but in this setup they have no reason to be powered.