One dietary source of magnetite found across the entire population would be cast-iron cookware and heating tables. Over decades of use, under the right conditions, a fair amount might be spalling from a pan down to these sizes through chemical and thermal action, especially under high heat while frying. Does it really discount any other environmental source of magnetite that the ones they saw _look_ like that found in microorganisms? There are vugs in volcanic rock that look like fossils, that doesn't mean algae have ever chilled their feet in cooling lava.