Ah... ya. perhaps. It seems I messed up with "reversed order of digits".

They both mean the same, 6 >= 5 >= 4 >= 3 >= 2 >= 1 <- descending order and that's the highest possible number, what's confusing about it?

hm...
task requirements sounds conflicting.
The "descending order of digits" requrement is conflicting with the "rearrange the digits to make maximum number" requirement!
It can be or descending order of digits or rearranged. Because "Input: 145263 Output: 654321" - that absolutely does not mean "descending order of digits".
So, requirements are looks confusing. It should be more clear.