In version 10.1 of the Installomator script, the installation type of XQuartz is set as
type="pkgInDmg"

that was correct for v2.8.2 of XQuartz;

however as of v2.8.3 it has changed to .pkg
(The latest v2.8.4 is also .pkg):

The XQuartz entry in the Installomator script needs to be changed to:

xquartz)
    # credit: AP Orlebeke (@apizz)
    name="XQuartz"
    type="pkg"
    downloadURL=$(downloadURLFromGit XQuartz XQuartz)
    appNewVersion=$(versionFromGit XQuartz XQuartz)
    expectedTeamID="NA574AWV7E"
    ;;