You must have misread - it says 84 x 48, not 84 x 84.
Yes, you can scale the screen - but it should be "nearest-neighbor" or pixel-perfect scaling, which means that if you scale down a screenshot of your game to 84x84, it should look identical (https://imgur.com/a/ggbBqEl - the original is on the left, the scaled-down version on the right). What you did is essentially make 1 unit equal to 10 pixels - but you're still rendering everything to a 840x840 canvas, instead of rendering it to a 84x48 canvas and then just stretching that canvas. (I also noticed that the meteors themselves have different shades of green in them, which is also not allowed - there should be only 2 colors on the whole screen at all times, no shades between one and the other.)
Sorry I didn't read the description for the controls - now the gameplay makes sense, it's actually quite interesting to control when the meteors come and then just reacting to them, although the coins always appear so close to the left that it seems to be just luck whether you collect it or not.