dakar

Not really, it just has to work on a few people.
With how cheap online ads are, if just 1% of people are stupid enough to act based on ads, it makes them worth it.

From your own source:
“For the 20mph sites (which are not thought to be representative of all 20mph roads), the average speeds were above the speed limit for all vehicle types, ranging from 22mph to 28mph but below the average speeds seen on the 30mph roads.”
So the average speed does decrease, increasing safety. Just because the effect isn’t a perfect 10 mph reduction doesn’t mean that it does nothing.
This means the proposal is effective, but it could be improved with traffic calming measures.

For anyone that doesn’t get how incredibly stupid this is, imagine Coca-Cola mandates all its workers to drink Pepsi.

With one post you won’t get anywhere, you need to put some more effort. I have my own community, which is quite a bit more niche than yours, but by posting everyday it has been growing slowly.
Yours has lots of potential but needs more people to find it. Most people find a community from a post and people tend to look at newer posts, so you have to put something out regularly.

Cars also make an economic barrier, poor people who can’t afford a car can’t get a job far away, limiting their choices and opportunities.

Even if they find a good job, companies prefer people with a car, so it’s very likeky that they’re rejected.