looks like the Americano series are equipped with tandem wheelset. That's heavy duty touring.
I believe he is trying to make a sale. If aluminum flexes, then the result will be a crack. If steel decide to fail, it bends from fatigue, aluminum just snaps when it fails. Steel gives you a much noticeable warning. So...steel bends. It flexes when you put a lot of stress, but when you are touring, it's not likely your going to go all out like racers do when they pursue. Steel will absorb more of the road vibrations and shock. You might want to try a different shop if all they do is pitch Giant brand. BTW Giant is THE world largest bicycle manufacturer. They build bikes for different companies besides their own. So you never know where or what brand your really riding. There are many touring bikes out, but if you go by manufacturers then it is only limited to a few by brands. There is more options when you want a racer. Remember when choosing a touring bike to choose one with a good wheelset. I like wheels with 36 spokes or more, they take the brunt of the weight (rack, pannier, other accessories).
owner of a early '80s Trek 620. Current offering for Trek is the 520