Taking passive aggressive parting shots is unlikely to make them more attractive to potential future employers. Even among employers who share their values, no one wants someone who has shown they're willing to publicly disparage the company.
That said, I think there are quite enough "openly woke" companies that it's not going to be hard for any of these people to find a welcoming home. Especially if they manage to successfully play the martyr on social media. This is where our society is at right now.
Overall this is probably a win/win -- the employees who left were surely the ones most dedicated to keeping the office political. Basecamp got rid of them and can rebuild with a team that's more aligned with the new policy, and those employees can find new jobs at companies that don't mind being political.