There is zero give, therefore the location of the top pulley does not change with chain tension. Therefore it shifts just as well with a minimal length chain as is does with a longer chain. Hence, it is insensitive to chain length.

You may be thinking of what happens if you put on a chain shorter than minimal length, that doesn't fit around big/big. Only an idiot would do that; when considering the impact of chain length on shifting performance it is completely outside consideration.

Also, the cage pivot IS sprung (you know, "single sprung pivot", as opposed to other derailleurs have two sprung pivots). If the pivot were not sprung, it would not be able to take up slack, obviously.