If you are viewing an image larger or smaller than its original size, filtering has to be applied for scaling regardless of Syphon or not.
Default filtering is GL_LINEAR, or a linear filtering model, not a pixel art esque GL_NEAREST, for nearest neighbor filtering.
Thus if you are viewing a 10×10 image at anything larger or smaller than 10×10, it will be filtered with GL_LINEAR filtering. Understand, the actual Syphon stream has *no* filtering applied as it is not being scaled at all – it is only on the onscreen viewing where filtering is applied.
Simple Client is not meant to be anything other than a basic QA test to ensure Syphon streams are being properly published and received and driven by the renderer appropriately.
If you want to force nearest neighbor filtering on your output, you have to do that via a feature in the application your are actually intending to use as a Syphon Client.