Why no vote equals a vote for Trump instead of the contrary a vote for Harris?
You are assuming that all people not voting or voting other party would vote for Harris, but this is probably not true.
Many of those people would vote for Trump if forced. Thus it would be better for the Harris campaign that they stayed home or keep voting for the Tea Party or some other third right wing party without representation.
Correct. These statements are always made from a position that the speaker assumes the votes Belong To Their Candidate. The fact remains that not voting for the orange hitler is still not a vote for orange hitler. Voting 3rd party is also not a vote for orange hitler.
No one may assume that another citizen's vote belongs to any candidate. Their vote belongs to them alone, to vote or not as they choose.
Having stated the facts of how that works, now I recommend that we all vote for Harris and against orange hitler.
This "every third party vote is a vote for $republican_candidate" noise comes around every four years like clockwork. Third party votes are not just protest votes from wayward Democrats.
It basically boils down to the same thing as the complaint "software piracy is theft" when talking about sales (or votes) that likely wouldn't have happened anyway.
The electoral college favors Republicans. Specifically it favors low population areas, which tend to vote Republican. High population centers tend to vote Democrat. Democrats need more votes to overcome this advantage.