Palatino 766 pipes, pools, cliffs, streams... (by Taccola)

[Koen G]

The influence of Renaissance in Siena MS may be a complicated issue...

I definitely agree. The Wikipedia article calls him a transitional figure, with his art style influenced by medieval manuscript art rather than Renaissance innovations:

Taccola's drawings show him to be a man of transition: While his subject matter is already that of later Renaissance artist-engineers, his method of representation still owes much to medieval manuscript illustration.[18] Due to the political rivalry between Siena and Florence, Taccola was never exposed to linear perspective, a growing graphical style in Florence.[19] Moreover, Taccola drew his machines based on what looked proper to him, not based on geometrical considerations.[20] Despite these graphic inconsistencies, Taccola's style has been described as being forceful, authentic and usually to be relied upon to capture the essential.[21]

So I don't know how to classify this use of parallel lines for shadows, but Taccola seems to have loved it. Sometimes his application of this cast shadow is not very elegant, and that's where it gets interesting. He will follow the outside border of the object that's casting the shadow, almost drawing like parallel hairs on it. This is a very specific technique (or misuse of one) that people would not use spontaneously.

The VM does this too in some cases, to draw these parallel lines attached to objects. And this is again why I suspect they had access to a Taccola manuscript. I'm not sure how well I'm explaining it, or if these images will be helpful... These are not meant as parallels for the whole construction, but just see how slightly awkwardly shadows cast by the object are drawn.

   

See the pillar held by the ship: it is "hairy" all around.