Correct - no battery needed, but the dummy battery keeps the tension and pressure on the capsule pin to stop it going crackily if tension was lost. In fact Sennheiser used to make plastic dummy batteries specially for this purpose.
I know the K3U worked like this, bus wasn't sure about the K2U which was before my time - the K2 series was replaced by the K3 in the 1970s I think.
The original batteries were discontinued years ago as they were Mercury batteries - but there were replacements around that did not use Mercury.