Whilst full blown FAS would most of the time mean slower development, the thing to remember is that foetal alcohol exposure can result in a wide range of difficulties or signs. There is more than one diagnosis on the spectrum, and just because a child doesn't have FAS (with facial features) doesn't mean that their brain hasn't been affected by alcohol. They may well still have FASD.
So yes, a child may develop well as a baby, and may still have FASD. Difficulties with social skills, academics/memory (to be honest, just because an 18 month old is 'bright' does not mean they actually do have a higher than average IQ, they are still too young to tell), some behavioural issues or many of the issues children with FASD might have, won't develop until they are older ....this baby is far too young for anyone to know yet whether they have FASD or not
That does mean there would be uncertainty. The SW is not right to say the baby must be fine because the baby isn't delayed. Not all children with FASD were delayed as babies, because there is no universal sign of brain damage caused by alcohol that all affected babies/children have.
My DD2 was exposed to a lot of alcohol in the womb. I don't know for sure how far it has affected her (although I suspect quite a lot, not all of her difficulties can be explained by trauma and mental/emotional health issues). The thing is, her difficulties are not things you would see in an 18 month old. She is not at all academic, and has a mild learning disability, but not a moderate-severe one, so that wouldn't cause serious developmental delay. Issues with understanding cause-effect, problem solving, social skills...again, those are things you won't start to see at a year old.
I do agree that FASD can be made to seem all negative, which is a real shame. If you think you can provide the support the children will need, that's fantastic. You don't know what your childrens limits will be, and FAS or FASD is not some absolute barrier to having a good life. Some children do very well indeed, it's a completely individual thing.