I noticed this option was missing from the template. When I followed the directions in README to enable it, I put it below line 59 without noticing it wouldn't have any effect by then. Prevent others from making a rookie mistake like mine.
@@ -19,6 +19,9 @@ ZSH_THEME="robbyrussell"
# Uncomment the following line to disable bi-weekly auto-update checks.
# DISABLE_AUTO_UPDATE="true"
+# Uncomment the following line to automatically update without prompting.
+# DISABLE_UPDATE_PROMPT="true"
+
# Uncomment the following line to change how often to auto-update (in days).
# export UPDATE_ZSH_DAYS=13