It’s not rocket science. The person I responded to said they want Harris to win. Thus they are a potential Harris voter. When they don’t vote, Harris loses a potential vote, not Trump.
Depending on where they live, this gets amplified by the systemic disadvantage of left-leaning states in the electoral college.
It’s not as easy as you make it out to be.
The Democrats have to try to achieve the impossible: trying to retain left-leaning voters while getting enough centrists/swing votes to overcome the systematic disadvantage the electoral college poses for them.
In a de facto two party system that puts them between a rock and a hard place.
But what does that mean for you as a (I assume) left leaning voter?
It’s actually quite simple: vote for the least bad option.
By not voting for Harris you may successfully show the democrats your discontent for their policies. But you pay for that by helping a possible fascist into power (remember: we already found out that not voting, helps republican candidates in most cases), who will be far worse on most policies you care about.