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.
The opposite of „not voting for Harris is a vote for Trump“ isn’t true because of the electoral college, which heavily skews towards rural states with not many voters, which are often conservative.
You need roughly 4 Californian votes to match 1 Wyoming vote. That’s why Republicans seldom win the popular vote and still manage to win elections.
So if left leaning people don’t vote (or vote third party), the negative effect for Harris is amplified in comparison to conservatives.
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.