The comparison function for sort must not just return 'true' and 'false'.
It must return '-1', '0' and '+1' for comparing 'Less Than', 'Equal To' and 'Greater Than', just like the two standard comparison operators 'cmp' and '<=>' (for stringwise and numeric comparison respectively).

Just using 'true' (ie non-zero, probably 1) and false (ie 0 or '') will lead to confusion.

If f(a, b) is 'false' then f(b, a) must also be false because a 'Equals' b - for whatever value of 'Equals' this sorting choses to use.
If f(a, b) is 'true' then f(b, a) must return '-1' (ie a true value, but *not* just true) because a is 'Greater Than' b in this case, and therefore b must be 'Less Than' a - not 'Equal To', or something is decidedly wierd.

You have to be consistent however you chose to do it.

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