I do believe in demons. I don't, however, see any reason to equate ET with demons. The Bible certainly does not make that connection.
I’d take a look at “Alien Intrusion” before I’d write it off.
If we hold that the Bible is authoritative and tells us everything about creation, man, sin, death, angelic and demonic— it’s a logical conclusion. Not all tactics by the demonic are told in the Bible, but can be characterized under general topics.