How to determine effectively whether my paper is in scope
I don't know of PC chairs who would deliberately "game" the acceptance rate for a conference, especially an important conference. On the other hand, they do want a lot of submissions. Often they want to build a variety of viewpoints and research into the program, so your "outlier" may actually be a good thing to them. A lot of submissions makes the reviewing more tedious, of course, but allows the program committee to build a more inclusive and "interesting" program. 

Of course the reviewers will give detailed feedback to the chairs and the conference program committee so the answer you got isn't indicative that you are more or less like to have the paper accepted. 

Some conferences have very narrow scope, of course, and I suspect that the answer from the PC would be different if your abstract didn't sound relevant to them. 

I suspect that you already have the best advice you are likely to get. 

The only caveat I have here is that if the conference is so gigantic that the PC can't really respond to such questions they just have boilerplate answers to all conferences. But usually, someone would take a look at it. Committees are generally big enough that someone will look.