No it’s stupid because stretching something generally means by using less like stretching resources by using less, or stretching a band by pulling it making it thinner.
That’s why it’s stupid because it contains none of the original meaning.
I can figure out in sports it probably comes from stretching out, like giving it a bit of extra effort, originally probably stretching muscles a bit further. But it doesn’t work in that context either.

So in short, it’s a stupid use that is erroneously used out of the original context and meaning where it made sense.
There is absolutely zero stretching going on in Harris increasing her lead.

Most languages have inconsistencies, but sometimes English seems like on big inconsistency, completely void of any logic.