Hard conversion is, as you said, a simple multiplication problem. For example, to convert inches to millimeters, multiply by 25.4. In the NEC usage, they will round off to the nearest, reasonable, degree of precision. For example, "6.5 feet," a number that appears in 110.26(E), is approximately equal to 1.981 meters. They rounded that to 2.0 meters. That is a "hard conversion."
A soft conversion is more of an approximation, and less of a pure multiplication problem. For example, you could convert one inch to millimeters, and get 25.4. But Table 4 (in the Annex) tells us a 1 inch trade size EMT conduit is actually 1.049 inches in internal diameter. That equates to 26.6 mm, as is shown in that same table. But the metric trade size is called 27, and not 26.6. So what is happening is that you are converting a trade size (1 inch) to another trade size (27 mm), and the multiplication is not an exact match.
The tricky part of the statement in 90.9 is that the "hard conversion" is from English Engineering Units to Metric Units, not the other way around. For example, for many years the NEC used 6.5 feet as the required headroom. They did not get that number by starting with a metric value, and converting to feet. Instead, when the recently moved to metric units, they did a "hard conversion" to metric, and the book now says "2.0 meters (6.5 feet)."
