At first glance I'd assume the idea is that a browser can choose to ignore author stylesheet font sizes if they are smaller than a certain size, ensuring text is always a minimum size of say, 16px. This is in contrast to approaches that vary the text size consistently -- either by scaling text alone to larger, constant sizes, or by using a "zoom" property. I can imagine that without JavaScript, a minimum font size would be harder to support and still maintain compatibility. This is why for most retina screen use cases, scaling is employed instead of font-size changes.
Getting back to minimum font size, I assume the point is that we shouldn't use em or rem to base our designs solely on font sizes, but instead on a combination of those and percentage-based units. Also, I imagine all layouts must "flow" to accommodate larger font sizes. Really, the biggest issue is that every browser and device supports "zoom" for text in different ways -- look at Android, for example -- and it's hard enough accommodating screen readers and multiple platforms to then take into account how browsers can change the page. To some extent, it's like complaining that your websites don't fit on my 800x600 CRT unless I make the text really tiny. Yes, it's a legitimate complaint, but maybe there's another browser out there or extension that can handle it -- e.g. "Readability" or Safari and how it can pull body text from a page. Or the ability to "zoom in" on sections of a page, e.g. with a magnifier.
It's a common(?) browser setting. E.g. in Firefox go to "Preferences > Content > Font & Colors / Advanced". It's useful for preventing websites from forcing font sizes that are too small for your display (e.g. in my case a 14" Full-HD panel).
Ideally no website would (1) change the base font size or (2) set fonts below 0.8em of base font size. Unless it intentionally wants unreadable text.
Alternatively I believe there are some high dpi related settings coming into the browsers to adjust the global "pixel size". That would "solve" the problem but really we shouldn't have been in this situation in the first place. Setting a pixel font size is just a stupid idea, obviously.
