In case anyone bumps into this and want a temporary solution, here is how I did it.

First, the Sketch component with ref:

<ExpoPixi.Sketch
   ref={ref => (this.sketch = ref)}
   ...
/>

and then, use the code section that @Nehero provided (i.e. https://github.com/expo/expo-pixi/blob/master/lib/components/Sketch.js#L165) with just the part below

if (this.sketch.stage.children.length > 0) {
  this.sketch.stage.removeChildren();
  this.sketch.renderer._update();
}

That did the trick for me

@victorcabeceira solution did not work for me. After further investigation, it appears that several features of this module depend on dependencies which are not compatible for the latest expo SDK versions (My app is running with expo SDK 34).

Since I only needed the clearing feature and the ability to save the result ( I used captureRef from react-native-view-shot to do this ), I came up with a temporary solution to erase the drawing with react hooks and conditional rendering to unmount the component and then remount it.

const [isSketchClearing, setIsSketchClearing] = React.useState(false)

// Gets called when a button is pressed
const onClear = () => {
   setIsSketchClearing(true)
   setTimeout(() => setIsSketchClearing(false), 50)
  }

const Sketcher = (
    <ExpoPixi.Sketch
      style={{ width: '100%', height: '100%' }}
    />
  )

The ExpoPixi.Sketch component gets unmounted for a very brief moment and then remounted.

<View collapsable={false}>
  {!isSketchClearing && Sketcher}
<View>

This solution works for Android both in dev and when testing the APK build.
I didn't test it on iOS since my project is Android only.