Exactly this. I think is the best part of this feature isn't the speed test but the latency/jitter/packet loss measurements. These will help a lot in diagnosing issues with ISP's vs WiFi clients, when running the tests side by side.The speed test run directly on the router will be handicapped by the router cpu.If you are getting full speed using the speedtest app then you are good.
I see alot of people complaining about this.
Speedtest app > internal router test.