You misunderstand the issue. No one is saying that the plants are diseased, but they could be, which is precisely why there are laws regulating imports.
A phytosanitary certificate is there precisely to rule out possible infections.
The fact that it is completely prohibited to import citrus fruits into the EU is very annoying, and it is also debatable whether it really has to be an import ban, but that is the legal situation.
That simply has to be said.
And once again, plants from China are not always infected, but without a plant inspection (phytosanitary inspection), it can happen.