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 (muscles), like giving it a bit of extra effort. But the headline 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 had a common denominator that made sense.
There is absolutely zero stretching going on in Harris increasing her lead.

Most languages have inconsistencies, but sometimes English seems to have more inconsistencies than any other language I know, and they are almost completely void of logic.