I don't think he literally meant it.
It is known since PS/SS that the Killing Curse normally doesn't leave any kind of physical mark, and most students knew it by the beginning of the 4th year.
It is not clear what kind of damage the Killing Curse does.
If AK damages or rips the soul, Moody should have said
you could all get your wands out now and point them at me and say the words, and I doubt I’d get so much as a one tenth of my soul damaged.
which is an odd statement, and wouldn't have conveyed in the same way the idea of AK requiring magical strength in order to work.
If AK kills by stopping all bodily functions, Moody should have said something like
I doubt I’d get so much as a brain cell not working.
If there is another mechanism, someone should have explained it in advance, and Moody's example would have likely been more complex to understand.