compare "reduced from a day and a half ....." to the " time......"

in the latter case, time explicitly states the thing which is being reduced.

i think this makes it clear

I would further venture to add the folowing:

If there was a comma to separate the clause "from a day and a half to 93 minutes" like this-
The automotive conveyor-belt system, which Henry Ford modeled after an assembly-line technique introduced by Ransom Olds, reduced, from a day and a half to 93 minutes, the required time of assembling a Model T.... then i think this sentence is correct.

However, if there is no comma, then reduced needs to be immediately followed by "what is being reduced", in this case "the required time of assembling a Model T"