It really depends what you mean by launch. Often it’s thought of as a big bang where everything is up and running at in one go, but it’s most likely to be a staged thing, where a testnet gets on its feet and stays there, and then groups of features are added, or become accessible over a period, and then it becomes a beta etc.
I guess the closest you might think of as a single launch point might be the genesis SNT supply being created. So yes it’s highly likely that client apps will come after this point. But as I say, it’s not a clear before/after, and there are lots of parallel strands.