That can be used as a freestanding pronoun (rather than a determiner) in phrases such as these quoted from the Longman Grammar of Spoken and Written English:
That's how they were doing it.
That's what I thought.
That's that's true that. That's what they do.
From: http://www2.gsu.edu/~eslhpb/grammar/lecture_13/that.html
What does That's that's true that exactly mean?
It doesn't make sense. It seems to be a misprint.
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