Is it a good sign if an interviewer says this

It's a standard interview question. When I applied to the RAF I was asked why them and not the Army or Navy.

most of the time they have set questions and never really ask their own on the spot. they probably ask everyone that - a company i interviewed for called and asked questions like am i willing to relocate so i think they ask these sorts of questions to ensure you'd accept an offer if they give you one.

they also might want to see how much you know about the company