Just some observations and thoughts; I haven’t dug through the source code to verify.
score appears to be stored as an integer and not a floating-point data type. It looks like it rolls over at around two billion, so I’m guessing it’s a signed 32-bit integer.
As far as high score, it should be stored in the browser’s data store along with the game. So, convincing MakeCode Arcade that you have a new game would cause the high score to reset. Clearing your browser’s data store, duplicating the game, or opening the game in your browser’s anonymous mode are a few ways that I can think of to open the game with an initialized state. Publishing a new copy of the game generates a new URL, so using that new URL should also convince MakeCode Arcade that you are playing a new game and give you a clean state.