2018 MacBook Pro won't let me install macOS High Sierra 10.13 from USB installer
Original answer

As described in this support article, there are different ways to load macOS Recovery: 


  
  cmd+R: Install the latest macOS that was installed on your Mac.
  alt+cmd+R: Upgrade to the latest macOS compatible with your Mac.
  shift+alt+cmd+R: Install the macOS that came with your Mac, or the closest version still available.
  


I suggest you try the last option and see if it lets you install macOS 10.13.



Update as per OP comments

Apparently the key combinations from my previous answer did not work for you and you receive the message "/Applications/Install macOS High Sierra.app does not appear to be a valid OS installer application.", when creating the USB installer.

It seems you face exactly the issue discussed here. The AppStore downloaded only a down-sized version of the installer image. Try the solutions offered in the answers over there. Basically you have to first download the full image / SharedSupport folder before you can create your USB boot installer.