Of course, I do agree with it. I can also confirm that going to gym regularly and getting jacked is not all that hard either. Nevertheless, median American is in terrible physical condition, obesity is rampant etc. Point is, there's quite apparently a wide chasm between knowing things, and actually restructuring your life according to those ideas: this requires committment, effort, planning, foresight etc. We gave up on expecting those things from average person in other areas (e.g. consider recent idea of "food deserts", which implicitly assumes that a person affected by those is unable to have enough foresight to make weekly or biweekly travels to grocery store, something our ancestors 100 years ago were perfectly capable of), so I find strange the incredulity of cycling proponents when it comes to understanding people shunning bicycles.

Similarly, many people have views about cycling that are false. For example, I've talked to several drivers about buying groceries on a bike, and I was surprised by how few seem to consider that you can add cargo capacity to a bike. Groceries seems to be many anti-cycling drivers "gotcha" question, but it's really not a problem.

There's no "gotcha". It's simply extremely convenient to do grocery shopping with a car compared to a bicycle, and adding cargo capacity only alleviates some of the inconvenience. The only reason people bring up groceries, or ferrying kids to school, or all the other excuses you've certainly heard fair share off is that when you say "car is simply more convenient to me", you're met with incredulity, and are being explained to, like the grand parent poster, that driving actually makes thing worse for you. You need to keep explaining the exact ways in which alternative ways are less convenient (less cargo capacity, more physical exertion required, more time spent on commuting with public transit etc), and the response is usually either minimizing the cost, or moralizing about health benefits or externalities.

I don't think these things are obvious, but most drivers I've talked about these issues with seem to take very little initiative to investigate these assumptions. The easiest way would be to ask experienced cyclists.

Indeed, "it's simply more convenient" is not the acceptable response here, you need to ask experts to tell you that you're wrong.

Also: I don't think that I'm calling drivers stupid or oblivious. If you think I am, can you explain how?

I don't think I quoted anyone calling drivers that, but the grandparent comment certainly implied that people driving in Manhattan are making choice that's simply wrong for them, which implies that they are either stupid, or, more charitably, oblivious.