"I guess the general US public can't really grasp small fractional sizes or thousands?"

The fractional sizes would get weirded and weirder as they get smaller.

1mm is ~1/32 (5/128ths is a really close approximation, but let's assume nobody is that crazy).

Right now it's ~1/4 of an inch Let's say they reduce the size 1mm.

Now which sounds better: 5.1mm, or 7/32 of an inch (or 13/64ths, which is more accurate)

We're selective in our use of the metric system here in USA, and therefore Americans will generally have an intuitive sense for how big metric units are for some measurements but not others. If you say 5mm or 1L, no problem. If you say 250km or 22kg, not so much. Americans have an intuitive sense for long distance in miles, small volume measurements in teaspoons or cups, distances in inches, feet, and miles. But micro distances in mm are generally okay, largely due to things like nuts and bolts being largely metric, and the proliferation of beverage containers in 1L and 2L bottles and engine displacements measured in liters give us a hand there.

Maybe it was intended for the Canadian audience, where metric for length, and imperial for weight, is common?

American here: we all were taught the metric system at least as far back as the early 1970s in elementary school, and I'd say even we don't know why the heck we still see the old English system in use.

mm makes it sound small

we don't understand grams or kg as a daily measure so we have no idea what .435kg feels like.