So what was Cuphead? I know, I know, it's not an FPS, but it could use the exact same title format and nothing would be different.
This game is only using an art style. There's literally no way this would cause any IP lawsuits even if it released today, unless you're actually playing as Steamboat Willy and the whistling tune plays somewhere in the game. Which, to my knowledge, isn't the case.
It's an interesting art style to give an FPS, but that's about all this is.
Edit: holy shit I just realized: this is an entire game based on GMod Nextbots. You're just shooting at animated PNGs that shoot back. Holy fuck.