most of that or most of them or most of it

I happened to read the following sentence in ‘Poached’ by Stuart Gibbs, which deals with coalas. They(Coalas) rest sixteen to eighteen hours a day and spend most of that unconscious. But I wonder whether the word ‘that’ is grammatically correct, for I think that ‘most of them(the resting hours)’ is correct. Would you grammarians teach me whether it is grammatically correct or not?

Yes, it's fine.

They spend most of that ^{i.e. time devoted to resting} unconscious.

Those hours allocated to resting may be understood by the speaker as a contiguous stretch or span, hence that rather than them.

That is not to say that one could not write "them":

They spend most of them ^{i.e. those hours devoted to resting} unconscious.

Both are grammatical.