No they couldn't have. Several people quit, they cried about Bezos blocking them from endorsing Kamala, other news outlets reported it.

All Bezos could have done in that situation would have been to say nothing but people would absolutely have been the wiser about it being blocked by him and they'd probably lose as many subscriptions as from this. I doubt his statement wins or loses him any meaningful number of subscribers, it's just cover so he doesn't have to come out and say actually he'd rather stay on Trump's good side in case he wins and/or maybe he thinks Trump is better for his interests but of course would prefer associating with someone so uncouth.