Making a virus identical to one which existed in the past isn't particularly worrying, because the original virus is already in the wild (except for the very few which are extinct, because of vaccination efforts).
The real trick is creating a novel virus, which our immune systems aren't all accustomed to already, and that's a whole different challenge. I don't think our genetic engineering technology is at a point where that's a realistic concern yet
Viruses mutate over time. So the 1918 flu has not been circulating for over 100 years. No one under 107 has been exposed to the virus that wiped out millions aka it's novel.
Surely if we can easily generate a synthetic version of an old virus, we can similarly create a vaccine?
Yes, it's the standard flu that we make a flu shot for every year. It's was just more deadly. Somewhere around a 3-6% lethality.
Swine flu bears such a resemblance to the 1918 flu that scientists believe it would confer immunity to those who were infected or inoculated.
What's the claim about terrorists about?
White Plague by Frank Herbert.