In fairness to Liberal Voice for Women, they've been fighting this issue for a long time, and the change now proposed has only come about because of their efforts. It's not perfect, and will need to be challenged further in due course, but it's got the Party's transactivists in a rare tizzy because they can see that the law has trapped them.
LVW have made clear that the "cis" language isn't acceptable (or legal). See for example this quote from The House Magazine: "A spokesperson from the group said that the new quota “guarantees those with a trans identity at least one place on every committee over 10 people, despite the fact trans people are only 0.5 per cent of the population and that there is no evidence they are underrepresented on Lib Dem committees.
"They added that “cis” is not a term recognised in law, and is “a label many women would reject and – given everyone has a sex – is unnecessarily exclusionary against those with trans identities”. "