It's definitely true-ish, and the examples are neatly worded, if still ambiguous without the eplanatory parentheses. And yes, there is potential for ambiguity, and it's usually necessary to consider the context carefully to be clear about meaning. This problem of clarity of reference for pronouns comes up a lot in English, and is often a source of humour, deliberately or otherwise. I suspect the "rule" is of the writer's own creation - but I don't know that.
As to interchangeability: "this" and "that" can introduce new topics, though they can also refer to something already mentioned, and "that" has other functions too. "It" typically refers back to the last-mentioned matching noun, except when it is used as a "dummy subject" I think.