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.

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.