This means the proposal is effective, but it could be improved with traffic calming measures.
The report goes into even more detail on this, the roads measured were primarily those without traffic calming measures. The overall subtext is that 20 mph roads should be built as 20 mph roads, including traffic calming as per the official recommendations. You shouldn’t just slap a 20 limit on a road built for 30 - which is what this post is about for Wales.
What they’re doing will increase noncompliance, not only in the areas where the road should be 30 but also in areas where it should be 20. It’s a cheap blanket change that’s more about political brownie points than actually achieving positive benefits.