No, actually it's never covered in the announcements that passengers should use the lavatory in their ticketed cabin for security reasons. That is not the language used in CO's FA manual at all.

From memory (sad, but true), I can recite the actual announcement:

"There are x lavatories on board this aircraft, and they are located throughout the cabin. The lavatory located in the first class cabin is reserved for our first class customers only, please. We appreciate you respecting this arrangement."

As an alternative, some FA's will give additional detail on the location of the main cabin lavs. 1/3 of the time, the announcement never even mentions the "reserved for our first class customers only, please" - and even then, 99% of the time, Y people go up and use the F lav and the FA allows it.

"Ticketed cabin" and "security" are never mentioned, although I've heard ONE FA bring it up (not following her authorized script). Alaska does mention this as part of their script - which I find rather annoying.

Now, is the OP's example really terrible customer service? I think the first FA should have handled it much better - so yes, there was a customer service failure here. A security threat? Not at all.