Presumably that was a blow moulding process.
If that was the case then the grease that you mentioned would probably have been a silicon mould release agent.
In any case it would have to be a food grade lubricant if the bottles were intended to be used for food, or drink containers.
This would only have been left on the outside of the bottles I assume for a blow moulding process.
The same thing happens in the manufacture of paper cups for Starbucks, and the like.
(Yes, I have worked in those factories as well)
The cups are left covered in a fine layer of oil (so fine that you would not notice.
Again it is food grade lubricant, that you could drink if you were so inclined.