Mike,
What I have found probably will sound like a wishy-washy answer.
In most areas I have tested the X-70 in, steel bottle caps came in as highly conductive targets; i.e., +46 or so. But, especially in the "99" tone option, they did not sound the same as quarters or halves - subtle difference but discernible. Was I right 100% of the time, no and I did dig a bunch but that was part of the testing.
In other areas I found that they pegged high and were either clipped, rejected entirely or by rejecting the last notch, eliminated that way.
So, is the X-70 infallible when it comes to rusted bottle caps, especially the deeper ones . . . . . no, but personally, I found that after digging a bunch, I can mentally ID and reject them if they become an overwhelming problem. If I am in an area where there are old coins amongst a lot of steel caps, I still will be digging most of them to avoid masking issues.
So, while this answer might not have been as cut and dried as one might have wanted, I hope it addressed your question.
Andy Sabisch