Playing an open-string whole note on an electric bass is a single action. Of course how you pluck the string - angles, speeds, pressures, positions and so on - will affect the tone, but it is still a single action, and you have limited control over the note once is it sounding.
Playing that same whole-note on a double-bass with the bow, and giving it full value, is a totally different proposition. The primary purpose of Simandl is the development and control of tone and intonation with the bow and the requisite left/right hand independence. As such, a large proportion of Simandl is not relevant to the electric bass player. Ideas of left-hand fingering are useful to the electric bass player, but the open string exercises are designed to remove the left hand from the equation to focus only on the bow. In those early exercies there is no suggestion/implication of the correctness (or otherwise) of using (or not) open strings when playing actual music. Nor is there any 'guidance' or 'rule' on when or why to choose one over the other.
IMHO/IME. YMMV