My guess -
I'm sure back then she used templates.
THe templates were sans seam allowances.
She traced the outline on fabric for EACH piece.
The drawn line provided the sewing line, so having a consistent seam allowance was unnecessary.
With rotary cutting and machine piecing, there has to be a common measurement.
And a lot of those newer techniques for machine piecing were backwards compatible, and have been adapted to hand piecing.
What I was going to say - - -