I don’t disagree with what you’re saying, but I don’t think they’re claiming that this move alone will prove a panacea. It still might encourage people to walk or cycle - not as many as the changes you suggest, but it could help.
Also this line:
Getting hit by a car at 20 is better than 30, sure, but it’s objectively better to not risk getting hit by a car at all.
At least if it’s going at 20 that reduces the risk of being hit at all (as well as the consequences if you are hit).