You’re right, that the net impact is exactly half of voting for trump instead - in that you only reduce biden’s vote, as opposed to increasing trump’s.
In less consequential elections, I absolute agree that voting for the candidate you want is key. But the risk here of a bad candidate is not just a shit period of government, but a genuinely significant risk of incurring a coup.
If you assume a default vote of “any viable candidate other than trump”, you still move in the same direction as voting for trump instead if you vote for a 3rd party candidate.
In countries with a healthier (not two party) system, this is typically the point where the centrist and left parties tend to form an electoral alliance to avoid the disaster candidate. That doesn’t really work for presidential elections though, so individual votes need to take on that same role to minimise the risk.