Google Analytics and any JavaScript based analytics can only ever be regarded as indicative.

If you think it measures your whole audience you're wrong. People might not finish load the page for example. Or they've ad-blocked the ga.js. Or they are using NoScript. Or they're using an accessibility layer.

It may be the majority of your audience, but I'm fairly sure that even with this add-on/extension that GA will still remain a good indicator of your audience size and your audience habits. That hasn't changed.