I'd agree with your concerns if they actually were creating really new life. The trouble is, that's just a marketing line. They're just doing what "genetic engineering" meant before the term got watered down. These days, genetic engineering means taking an existing life form and adding or modifying one or two genes within it. The impression I got when I first heard about the concept (and which you seldom hear anymore) was that of considering the entire genome of an organism, deciding what can stay and what should go, and what the removed stuff should be replaced with. I get the impression that "synthetic biology" is just a term trying to get back to that original concept.