The face-hugger? Yes and they had no reason to presume "Kane's son" was any different. As I said, Dan O'Bannon was pretty explicit in on of the DVD interviews that he didn't like the idea of some invulnerable beast where the bullets would just bounce off it. He wanted it to be a plausible, logical and extremely dangerous animal, not some supernatural terror.
So my statement still stands. As far as the crew were concerned, the alien was unkillable, unstoppable and uncommunicable. It was only in the sequels that it had all its teeth pulled. Whatever someone said on an interview, similar to all the statements the Great Man made himself after Blade Runner, is immaterial to the information we are given on film.