I know it’s not your argument, but this still doesn’t make much sense to me. They are basically arguing that infrastructure is zero sum, where we have to choose between bikes or public transit?

Okay I will say at this point though, infrastructure is basically zero-sum. That's exactly the reason why you can't have a transportation system that caters to the needs of car-brains, pedestrians & public transportation all at the same time. There is a finite & largely fixed amount of physical space that you can allocate to both commuters (of all kinds), and localities for use in whatever they need to do, and various forms of automated locomotion (cars, trains, busses) all take up a lot more space than walking or cycling somewhere.

I agree with 7bicycles conclusion though, btw. But the thing you're saying here is not the part of the argument that's wrong.