It won't lose them the lead though. That's why this decision has been unofficially leaked and denied for a while now. The Labour party have conducted polling in that time and seen that this decision doesn't really register for the majority of their voters. Not all of their voters but enough that they're confident in dropping this policy.

It's how all parties operate, you'll notice a pattern.

1. Leak story.
2. Claim you don't know about the leak and won't publicly comment. If asked give some fluffy answers.
3. Get your research / polling department to gauge public opinion maybe via proxies: How would you feel of this rumour about policy X is introduced / dropped?.
4. Figure out if this policy change will hurt you, be net neutral, or win you votes.
5. If the change in policy hurts you then say this was a malicious leak and we do not support it.
6. If the change in policy is neutral to you then say yes well unfortunately we will have to implement this change but there's nothing to see here let's move the news cycle on.
7. If the change in policy is positive say thia was a leak that we don't support because we were working on bringing proper communication for this out in the future but now that you mention it... yeah we'll take credit for it.

Ultimately Labour voters need to be a lot more vocal about not voting for Labour if they continue to abandon green policies. But maybe because of the disenfranchised nature of our politics or that they just don't give a shit about the environment they clearly aren't going to stop supporting Labour because of this.