Yup, I thought 'bronze for alloy'was the usual rule of thumb, but 40 years of continuous running with AMC twins alongside whatever else I've had has taught me otherwise. Iron guides in alloy heads throughout the production, very long life, low rates of wear, tight stem to guide clearance - that mix of bits can work perfectly OK.
Maybe due to good design and materials, good machining standards and good geometry, dunno - but those engines rev, to 7000rpm mostly. Of course, some might say 'huh, they need to, if you want to get anywhere' . . . but that would be a bit harsh, as they like revs and they do get there. But, to paraphrase, AMC is another place, and they do things differently there.