I'm using Android. I think there's no logic that it's related to the user agent. Immediately after I disable the Ninja Firewall the comment line will appear.
I've tried seeing it through the http://www.ready.mobi/ and the same thing happens:
- with the Ninja Firewall enabled: there's no Quick Cache comment line
- with the Ninja Firewall disabled: there's a Quick Cache comment line
Also I tried disabling some options from the Firewall but it didn't help. Only when the Firewall is disabled the Quick Cache line will come back.
I think I didn't say it's the Multisite installation. Actually I have 2 Multisite installations as a test environments and the same issue appears on both instances.
I didn't activate Ninja Firewall on regular WordPress yet (without Multisite), so I cannot say about that.
Have you tested this in Multisite environment?