And the only intent was to tell the reader that even though you pass the LOOKIN arrays in sequential order, you still can reverse its order by reversing the order of numbers in the first argument array of the CHOOSE function.

Which means that if you pass the LOOKIN array in the desired order already, you don't need to reverse the sequential numbers in the index argument of the CHOOSE function.

3rd point. CHOOSE({1,2},A:A,B:B) doesn't pass the element to VLOOKUP as A:B, rather, it passes a combination of multiple arrays having 2 element each, one from A and one from B. The VLOOKUP function actually looks up at these individual combination of arrays.

Which leads to the point that if one were to write VLOOKUP(LookupValue, CHOOSE({2,1},C1:C10,A1:B10),3,0), the formula would return a reference error, because CHOOSE (or Excel) isn't actually combining the array in to a range, but in to multiple arrays