Source: p 258, A Student's Introduction to English Grammar (2005), by Huddleston & Pullum
[1.] The main elements that trigger inversion like this include:
... expressions containing so or only, as in 
- or Sue is going, and
soam I. ...
I ask this question only for 'so' in a correlative conjunction (eg 'Just as ..., so ...'), for which I usually witness subject-auxiliary inversion.
Thus, for the main clause, why were so and is NOT inverted?
I would've written:
[3.] ... so is protensio derived ...
[2.] For just as the word protasis is derived from a Greek verb of holding out or offering, viz., proteino, so protensio is derived from the matching Latin verb protendo.
Footnote: I encountered this quote while researching the etymology of 'protasis'.