Hi alixluohao,
I didn't read the paper you cited but I can try to help you.
The input referred noise is the same in both cases - i.e. open loop and closed loop- because - let me say-  the global "transfer function" is different.
The closed loop is able to reduce the output noise but it changes also the gain in the same matter, so the input referred noise is the same.

My considerations are taken on a generic system but they should be applied also for delta-sigma.

I hope to be helpfull.

Bye,
MarcoC