You hit the nail on the head........
Atwood has a small drain opening to begin with ---1/2"
Add a valve/hose etc and you have made the drain outlet even smaller.
Yes the valve is smaller inside regardless of the large outlet.
Set up makes for quick/easy draining of WH Tank but scale/deposits reman behind.
Don't know why folks think it is so difficult to remove drain plug, power flush WH Tank and then put drain plug back in.
I 'power flush' by turning water supply back on after drain plug is out.....shoots water and debris out.
Don't even mess with one of those wands...they just get in way cause they obstruct drain hole.