We have much better stuff than 'high power microscopes' now, field electron microscopes, atomic force microscopes, neutron scattering scopes, high power electron microscopes that can visualize individual atoms.
So claiming they didn't know what it was did not have access to modern equipment.
Elecron microscopes with "EDX" sensors for instance, I was lucky enough to have worked with the scientist who invented that technology, brand new to me in 1970 when I was working on Apollo as a tech at Goddard Space Flight Center.
EDX is a normal electron microscope with a variable angle X ray detector that ATT would be tuned to one species of stuff, aluminum, chrome, nitrogen, whatever, but now they can scan for atom and molecules up and down the AMU numbers. AMU for argon, for instance, is about 40. Arsenic clocks in at 75, Boron at 10 and 11 (isotopes) and so forth.
So if that sliver was saved, it could be inspected atom by atom, both by surface techniques like atomic force microscopes.
Just saying, they didn't have modern tools and undoubtedly came to unfounded conclusions.