I also suspect that in old woodcuts you needed to do that for the integrity of the image. I'm not sure a wood carving of a simple outline would survive multiple pressings as well as one with these hash marks.
isolated lines certainly do work, look at the frames (not on wasted molds like the Conver of course where everything fell apart) and note that when some lines are missing on older decks, it's more probably due to a failing inling than to a broken mold.
A single line would work because apart from a very small height where the line is isolated on the top of the mold, the wood is cut with a smooth slope that provides rigidity and durability.
Also note that it takes more time to cut a large area of white than making multiple lines (so it's in favor of not isolating lines), but it takes even more time to cut complex and precise interlacings, not hatchings but the kind of things you find in the Épées and Bâtons numeral cards - when it's properly done, some engravers chose to make it more "freely" or lazily, on this specific point this is one of the rare occurrence where my favourite old deck is not a reference, though some people argue this was done on purpose (but the same people believe any card details such as bad inking or wrinkle to be done on purpose).
Bertrand