I’m sorry, but I have to interject something here. I am not a great engine builder like Monty Smith and others on our forum, and I don't pretend to be. So I might be talking out my a$$ here. I'm just trying to help a little.
I’m not a metallurgist, but I know quite a bit about metals. My experience has been when you take similar metals and rub them together they start to gauld up. When you use a steel block with steel main caps on a race motor and you will still see the fretting, even if you use a girdle. It is the harmonics. They still move around. So when you have a steel block and aluminum main caps, I don’t feel that the aluminum caps can stop all the harmonics. What you are seeing, is the benefit of two Dis-similar materials at work. Two dis-similar materials wont gauld up when rubbed together.

This brings me to having an aluminum block with aluminum main caps. And with that combo virtually all those kind of harmonics are gone. The aluminum block is absorbing most of the harmonics, and wont gauld the mating surfaces between the block and caps.
Just an outsider looking in