I assume they meant that they default to making things that come in imperial-unit sizes (e.g., quarter-inch nuts instead of 6mm ones or whatever). It's not really practical to mix and match.
McM is just a warehousing and distribution company; they don't make any of the things they sell. They define products by specs rather than manufacturer (is few brand names are used in the catalog/site), which makes it much easier to stock parts of every nearly every flavor (imperial, metric, dozens of odd pipe threading standards, thousands of ASME/ANSI/DIN/other-standards-body-standards).