Halliday & Matthiessen (2004: 464):
There are different ways of referring back to what is quoted and what is reported. Typically a reference item, usually that, is used to pick up a quoted passage, while a substitute, so/not, is used with a report. … This is because the act of quoting implies a prior referent, some actual occasion that can then be referred back to, whereas in reporting there is nothing but the reported text. This explains the difference in meaning between I don’t believe that ‘I do not accept that assertion as valid’ and I don’t believe so ‘in my opinion such is not the case’.