This is how I'd say it: "Writing isn’t about picking the right topic, what you write about doesn’t matter that much." "As much" means something similar to "approximately; nearly" "That much" means something similar to "to a certain degree".

They are subtly different. "As much" (in this sense) would NORMALLY be in comparison to something else. For example, "It is important to write interesting characters and dialog. What you write about doesn't matter AS MUCH". "That much" (again in this context) is more absolute. It means that the subject that you write about is not particularly important. It isn't necessarily comparing the subject to some other specific aspect of the writing the way "as much" would. However "as much" COULD be, and often is, used as a substitute for "that much". Both would be understood.

句子里用that much，as much as....是一个词组，表示和。。。一样多，放句子里语义不通。