With dual-frequency in use you should see decimeter-level precision in the best case scenarios (outdoors with clear line-of-sight), although I haven’t seen any hard evidence from Huawei or Xiaomi devices that they are currently capable of this level of accuracy. As mentioned in the article there haven’t definitely been some implementation issues with properly supporting dual-frequency in the Android APIs, and I don’t know if there are also implementation issues (e.g., antenna design, algorithm issues) that are preventing dual-frequency from performing to it’s potential.
Please note that the “accuracy” value you see in GPSTest comes from the GNSS hardware — it’s not a measured value, and therefore can be very inaccurate itself. It’s the 68% confidence level, which means that the true position should be within a circle defined by the calculated position and the radius of the accuracy value 68% of the time — which means it can fall outside that circle 32% of the time. I’m currently working on adding a simple benchmarking feature to GPSTest that will actually measure error based on a user-entered location. In the meantime you could check out another app I created a long time ago for this particular purpose, GPS Benchmark (https://play.google.com/store/apps/details?id=com.gpsbenchmark.android).
Google Maps uses Google’s fused location provider to show position information, which includes data from other sensors (e.g., accelerometer, gyroscope, Wi-Fi) when calculating a position, so the position you see may differ from the raw GNSS location calculation you see in GPSTest. You can benchmark the fused location provider in GPS Benchmark too.