This is an ego fluffer app, not a scientific study. The accuracy is "good enough" to give you a rough idea of your performance, and if youre really into optimizing WPM, give you a way to track progress or the impacts of using different keyboards.

Getting a high score doesn't actually mean anything except you probably use your phone too much.

My Android keyboard is probably lacking the proper dictionary

I think using the dictionary is cheating here.

and is probably less reactive than the native app installed by default

That's weird. When I install keyboards I don't notice slowdowns but rather improvements due to the mainstream solutions being extremely bloated.

Even a typing test on a keyboard is odd. My grandma surely gonna fail this test and not because she's old, but because she doesn't know where the different keys/letters are placed on the keyboard.

But that's what the test is about. It's not about how fast you can press keys. It's about using your key position knowledge.