They will tax, not heavy tax, it's based on the countries difference to the EU average.
For the beginning, China has pretty good clean energy policies for stuff like aluminium and iron. Their export of just iron and steel is around 6bn EUR, and tax estimate on that is around 200m or so (3-5%), so impact won't be that huge yet.
Note it's a ramping scale over the next decade, and at the beginning only certain goods will be impacted. Will properly see cost impact by end of year, since from October it's just going to be disclosure of amount and not have to pay yet.
Overall it's a good thing, allows the EU to compete better, and encourages the rest of the world to go more green, besides the fact that renewable power is just cheaper, so everyone is heading in that direction anyways.
Mantashe comment is just stupid, since South Africa can compete better with renewables (way better sun, wind, etc.), has to build less for the same output as others, so if actually transition, it would be a competitive advantage against a lot of the rest of the world.