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."?