You do concede that people (including Thiel) can be decent and support Trump? I know plenty of good people who support him and for good reasons.
This isn't about the decency or not of Trump supporters. It's about indirectly supporting Trump and thereby increasing the chances he'll be elected.
Altman's and YC's non-disavowal of Thiel supports that outcome. Altman's support is a layer removed from writing a check for Trump, but it's still support.
If Altman believes (as he seems to) that a Trump presidency would be very bad for America, it's hard to see why he would not specifically repudiate Thiel's actions. That's different from repudiating a friendship.
If you had a friend who used their considerable resources to publicly and materially support what you believed to be a truly bad outcome for America, would you not consider saying, "I can't go there with you, and I can't let my company's prestige support you while you do."?
Any more would send the wrong signal and establish a odious norm in our free society.
Altman has made it quite clear that he endorses neither Trump nor Thiel's support of him.
"Endorsing" and "refusing to purge" are two different things entirely.
Do you really see no difference between the two?