Got to disagree with you, cannons and guns don't really require precision. US MBT's have smooth barrels which reduces accuracy (UK MBT's will have rifled barrels). In the olden days, you make a cast, pour the molten metal into it, wait for it to set. There is no need for measurements to the millimetre. The metric system (as we know it) really didn't come into use until pretty much the end of the 18th Century, the Musketeers (known for their, err, muskets) are from the early 17th century.

Modern weaponry will require precision, as they have precision parts. On the other hand, do we even know what sort of measurements Garl uses? The UK is a mix of metric and imperial, mainland Europe is metric, the US is imperial, so you can have disparate systems and they could actually use a standardised form of measurements which goes up or down by 10.

/edit - also, the bird men can make an airship, and they don't even have proper thumbs.