Strong Towns is much more focused on NOT building new roads than on building new transit. Overall, the message is that new infrastructure of all kinds can be unsustainable financially.
Unless your town is so small that you can get away with purely walking, you're gonna need either roads or transit.
But the denser you are and the more mixed-use areas you have, the less road mileage you need per resident.
They don't advocate having no roads and transit...just being more cautious about expanding whatever infrastructure you already have. They are a foil to the idea that infrastructure expansion is always an "investment." Their emphasis is on maintenance and staying in good financial shape so that you can afford to replace infrastructure when it wears out. It's actually not a very radical position which is why they're able to draw crowds in red and blue places both.