In the diagram, there are just two directories, and mount points could easily handle that case. In the video he describes how the scale of things makes using mount points here unwieldy.
/AppleInternal Device/AppleInternal
/Applications Device/Applications
/Library Device/Library
/System/Library/Caches Device/System/Library/Caches
/System/Library/Assets Device/System/Library/Assets
/System/Library/PreinstalledAssets Device/System/Library/PreinstalledAssets
/System/Library/AssetsV2 Device/System/Library/AssetsV2
/System/Library/PreinstalledAssetsV2 Device/System/Library/PreinstalledAssetsV2
/System/Library/CoreServices/CoreTypes.bundle/Contents/Library Device/System/Library/CoreServices/CoreTypes.bundle/Contents/Library
/System/Library/Speech Device/System/Library/Speech
/Users Users
/Volumes Device/Volumes
/cores Device/cores
/home Device/home
/mnt Device/mnt
/opt Device/opt
/private Device/private
/sw Device/sw
/usr/local Device/usr/local
/usr/libexec/cups Device/usr/libexec/cups
/usr/share/snmp Device/usr/share/snmp