He defined mission-focused in the original memo pretty clearly and succinctly.
> What is the scope of our mission?
> Coinbase’s mission is to create an open financial system for the world. This means we want to use cryptocurrency to bring economic freedom to people all over the world. This is difficult and important work, and every employee at Coinbase signed up because they are excited about this mission.
It's not operating with total blinders on the wider world, becoming profitable as a startup often requires having a vision of positively changing the world. Armstrong wants people who care about the mission quoted above, period. There are people who are passionately committed to this, yet have totally different opinions on abortion, marginal tax rates, racial politics, bail reform. There's no reason to think you can predict any of these for an individual based on adherence to the mission statement.
Achieving change on the one thing that matters to everyone also requires that you have viewpoint diversity on the things that don't, because your current and potential customer base sure does. To the extent that mission creep enforces conformity across a range of issues, you narrow the range of people who would be comfortable working at your company to an increasingly small pool, and you limit access to the sort of insights that lead to a better product.
The alternative is to have separate pillow companies for conservatives and liberals, which is ridiculous.