I'm not an economist, but I think that the net zero inflation figure came from:

- gold/silver standards of the time (disinflationary)

- currency in bullion or coinage of a relatively fixed qty (disinflationary)

- low credit supply & banking industry (not inflationary) - labor supply growth (deflationary)

- productivity growth (deflationary)

- war/crises (often deflationary)

There was probably some significant debasement and other funny tricks by the rulers of the day to intentionally inflate (state) purchasing power, and obviously there was some inflation via gold/silver mining and trade from the East. These combined with the above probably netted out of the long time-span.

It's not like they had bitcoin or central banking policy. The West didn't even have paper currency during this era; they didn't have access to inflationary levers to pull.