Personally, I'd argue that the 'Big Chap' in A L I E N has been understood to be a 'bio-mechanical' creature by many fans over the years. Of course that's due to the look of Giger's art style...but it's not as if Ridley asked him to tone that aspect down during for the final designs.
Sure, the 'egg' and subsequent 'facehugger' stages look purely 'biological'...but we definately end up with METALLIC teeth by the time the 'chestburster' arrives, and it's not unreasonable to assume that all the 'tubing' design details on it and the eventual fully-grown 'Big Chap' are somewhat BIO-MECH in nature.
I always looked on it's 'bio-mechanical' traits as just coming to the fore at that mid-way 'chestburster' stage, and along with it's 'acidic blood', it all added up to the 'Xenomorph' being a VERY 'alien' creature indeed.
And I certainly don't recall Ridley ever going on record to contradict this same general assumption by many others, concerning this creature's overall 'biology'.
As far as I know, the creatures in ALIENS, ALIEN3, and ALIEN RES all kept the metallic teeth too...cementing that particular feature into their look. But, on the other hand...Cameron chose to give the 'Queen' itself some 'translucent', non-metallic teeth...so that's a bit of a conundrum I give you - however I choose to just 'ignore' that particular anomaly as not being important in the overall 'bio-mechanical' scheme of things - at least they looked dangerously sharp...unlike a certain young 'Deacon's 'enamel' dentures, which looked disappointingly un-threatening.
The fact is, between Ridley's original onscreen 'Big Chap' and his otherworldly 'space jockey', things DID look decidedly 'bio-mechanical' in nature overall...and if EVERYTHING continues to point to a purely BIOLOGICAL make-up where his creatures are concerned in these PROMETHEUS follow-ups...then that leaves me with a big 'discontinuity' issue to overcome with my way of looking at his original movie's creature.
...which will just increase my contentment of choosing to look on these 'prequels' as merely being an 'alternative', expanded universe type of storyline, unconnected to the original movie's 'actual' backstory whatsover.