No, it isn't that I accept the proportion of women to be lower, it's that in the real life context, the proportion is even lower than this and the timeframe short. Turning it around to 40% men is irrelevant as you aren't starting from scratch, you are probably starting from 20% or so.
If we "turned it around," and insisted 40% of teachers had to be men, within 6 years, can you see that it could only happen by getting rid of more qualified female teachers?
You have to assume a lot of the senior roles will be filled be the same people, in six years, or people already in senior roles moving sideways. Women will be moving up and space is only created gradually. If we insisted that men were demoted/fired to make it 50:50, I think that'd do more harm than good (especially for the women taking their places).
40% by 2025, then make a new target of 45% by 2030....
The 20% is daft, but if it does just fill up with males, that will at least demonstrate an important point. It'll also be interesting to see which category TW go into. If they are counted in this "third space" instead of in the female group, then that's good.