"but I receive zero tax benefit"

You see that $100 is offset; that's the benefit. The rest will carry forward.

"As a comparison, accumulated capital loss only gets reduced if there is a benefit"

Certain amounts are considered Refundable = you get the actual money back.

Certain amounts are considered Creditable = they get applied to something, when there is that something as an Activity.

What you describe is, you didn't have to report the $100 Gross, because the credit offset it to Net $0. And now there is only $900 credit remaining. It's right here:

"$100 will be subtracted on form 4952 leaving $900 to be carried over"