I think this is because eslint defaults to files with a .js extension. Since our script takes in file globs we could just pass globs that include the .ts extension as a bin/eslint-unused-modules src/**/*.ts.

I looked into this a bit though and was hoping we could drop our custom solution for something built into TypeScript. I didn't find anything build into TypeScript but I did find this gist that seems to yield correct results. I also timed the gist against our solution and the gist clocks in at around 13 seconds whereas our solution takes 50 seconds.

I think github/github#140799 is a good indicator that we might want this check still.