"Surely only goods sold to the EU have to be in line wit EU rules?"
You might think so.
The (perfectly reasonable - or should I say "formal") requirement that goods sold to the EU must meet EU standards is not the whole story.
The "standards" themselves may be designed to protect EU toddlers from getting highly flammable, toxic Chinese teddies imbedded with needles as a Christmas present. And nothing wrong with that. Or the "standards" may be designed to protect inefficient EU manufacturers from competition. (Isn't that Trump's point about American chicken exports?)
But the rules go further. "Fair trade" EU style means that we trade freely with each other, but impose tariffs on those who export to us. So, if Britain outside the EU wants to export a product X to any EU country, totally compliant with the rules of safety, efficiency or whatever, but which uses components Y and Z bought from China at cost price minus the EU tax, then Britain will be breaking the rules of the EU's Customs Union's rules of origin: buy Y from a EU supplier at face value, or buy Y from China and pay a tax (which we have determined) on it.