To be fair, this post seem to be a way to tell a story about how the white paper came about. The white paper itself is actually a separate publication, which should be read with a more rigorous approach.

PCB design is unfortunately a confusing mix of metric and imperial measurements, and mils are a very common measurement in the field.

Unfortunately, imperial measurements are a pretty common thing in the EE field when it comes to boards. A lot of parts are standardized on the 0.1 inch grid that was relatively common for a good 40yrs or so. I think the commonly understood root is the Dual Inline Package (or DIP) that most early ICs used, which was on a 0.1in pitch for a fair time.

Many of the more common packages under JEDEC [0] are all defined on imperial measurements.

Some package footprints can be approximated or closely matched with round metric measurements, but when the part itself is designed to an imperial measurement you "go with the flow".

The "mil" is one one-thousandth of an inch (0.001) and is thought to have originated from it being a "milli"-inch. I think it's an artifact of decimalizing the inch, from what I've read in the past anyways.

[0] https://en.wikipedia.org/wiki/JEDEC