What was the "obvious" threat exactly INTRUDER? My interpretation is that something said innocently by a French national struggling with the English language was (wrongly) perceived to be a threat by an American crew.
Presenting a prepared note of that type to the flight attendant is NOT typical of "innocently... struggling with the English language." It is more typical of, as others have pointed out, "probing" or other reconnaissance.
I tend to believe the threat was real, even if not immediate.