If your goal was to get likeminded people who don't want to discuss politics into the company, and "pot stirrers" out, seems like the post succeeded. Maybe that wasn't the intent and they were naive enough to post something publicly like that and not expect resignations, I couldn't say.

Frankly, at this point I’m inclined to agree with OP that the politics rule was just a pretext to assert their power over the employees. (Partially because the employees learned about all this via the public blog post, which seems uncontroversially dickish no matter how one feels about the actual changes).

I’m sure they expected attrition — hence the severance — but I’d wager they thought it would be closer to Coinbase’s 5% when they announced a similar policy, not 30% and counting.

I don't think they expected to lose 30% of their workforce. That doesn't seem like something you'd do intentionally except in dire circumstances.

30% is like ~a dozen employees

Twenty so far, actually. Including Sam Stephenson, who seems to have been their longest-tenured developer. Also their head of design, head of marketing, and head of customer support. And their entire iOS team.

Yeah, sounds like a pretty big fuck up then on Basecamp's part. Weird that they didn't see this coming, I can't imagine being so out of tune with a large portion of senior, long-standing employees.