After thinking about how icons are used on desktops, in installers, in about boxes, by launchers, and in other ways I decided that BY-SA wouldn’t affect those uses (I am not a lawyer, and I am open to correction), so I wouldn’t cause any problems by confirming that the licence allowed this use.
I emailed back to say that I thought that the licence should cover the creation and use of a derived work as an application icon, and asked if the application was free software. They said no. I had deliberately waited until answering their query before asking this, as I didn’t want to appear to be considering it in my decision.
Sometimes free software and free culture do end up as separate magisteria. They shouldn’t be, but the details of how we support them don’t always ensure that they complement each other in every situation.