It doesn't usually get that cost saving until recent years (90s+)
I wouldn't go as far as to suggest she reused the thread - I've only done that once, with a red nylon thread I knew I wouldn't be able to get again, and I pulled off the entire overlocked hem's thread in one piece after it got snagged, to re-sew it onto the sheer skirt again. It's far too much trouble for most. They likely just found a good match. Having just resewn a mid century dress that was altered at least 3 times, I can appreciate the lengths some people go to to match a colour (nutcases like me) I was removing three different black threads, the only difference being one was slightly reddish and another slightly thicker.
It's not just that frayed edge that makes me suggest it was shortened - the hem of the skirt dips down lower on either side, being bias this suggests to me that it was laid out flat and cut across in a straight line, rather than on a person or in the round, which would result in an uneven hem.