It does, it just doesn't close all attack vectors used by APTs.
They say[0]:
> Turning on Lockdown Mode [...] further hardens device defenses and strictly limits certain functionalities, sharply reducing the attack surface that potentially could be exploited by highly targeted mercenary spyware.
They don't say "turn this on and you'll be unhackable". They go on to say:
> Apple will continue to strengthen Lockdown Mode and add new protections to it over time.
So what they released in the current beta is just the start. They decided that releasing Lockdown mode with only some additional protections would be worthwhile to at-risk users and I personally agree. It's both true that Lockdown likely helps at-risk users (see reply by _kbh_) and still has lots of room for improvement.
[0]: https://www.apple.com/newsroom/2022/07/apple-expands-commitm...