Apple appears to be banking on software efficiency gains offsetting the reduction in battery capacity. They've done this with various iPhone and iPad releases several times.

But the trouble with trying the same thing for the Mac is the wider variety of software people run on macOS. Third-party software may not have got any more efficient, meaning the battery doesn't last as long.

But if you stick to Mail.app and Safari then, in theory, you'll get the expected battery life.