You are right about "this" refers to something is going to be mentioned. But we can use both "this" and "that" (and actually "it" too) to refer to something that was mentioned earlier too.
Here is an excerpt from a couple of entries in Practical English Usage by Michael Swan,
590.1 referring back
This, that and it can all be used to refer back to things or situations that have just been talked or written about. It does not give any special emphasis.
This and that are more emphatic; they 'shine a light', so to speak, on the things or situations, suggesting 'an interesting new fact has been mentioned'.
"So she decided to paint her house pink. This/That really upset the neighbours, as you can imagine."
This is preferred when there is more to say about the new subject of discussion.
"So she decided to paint her house pink. This upset the neighbours so much that they took her to court, believe it or not. The case came up last week ..."
590.4 referring forward
Only this can refer forward to something that has not yet been mentioned.
"Now what do you think about this? I thought I'd get a job in Spain for six months, and then ? (NOT
Now what do you think about that/it ...)"