Why wouldn't they allow keyboard input to type the words? Nobody has time to select each individual letter with the up/down arrows. I get it's an emulated game but there must be a way to hook keyboard input.
As the other person who replied said, it's a game meant to run on the Game Boy which has no keyboard. The intended use case is not in a JS web emulator, that is just a way for people to preview it before downloading.
You can also see in the downloads there IS support for the game boy clone megaduck laptop that has a keyboard, if you have one of those.
Certainly possible and probably not too hard, there are code examples from the late 90's and early 2000's for using ps2 keyboards. But not enough people have the keyboard link port adapter to make it worth implementing.