While that makes sense, this is hardly a new discovery. This was known when Apple switched to Titanium with the 15 Pro, and it was also known when Apple switched to stainless steel with the iPhone X.
I doubt they'll be like "we discovered aluminum is lighter than titanium by volume, great, huh?". I think they'll either talk about how the aluminum in the 17 series is some kind of very special aluminum ("grade 15 ultra pro max aluminum used by NASA") or they won't talk about it at all, simply marketing how light the phone is (17 Air) without mentioning materials at all.