You might not see a recipe for canning sweet potato leaves because it's not common to eat them - or maybe because canning them would result in a disgusting mush product because they are extremely tender leaves. You don't see recipes for canned nasturtium leaves either- not due to safety, but because it's not common to eat them and they are a fresh eating leaf, not a canning leaf.

The processing times for sweet potatoes themselves is in a pressure canner. Pressure canners remain sealed during the processing time, it's the nature of the pressure canner. So only a little water is needed.