Let me spell it out for you.
Someone posted that he couldn't float. You felt the need to tell him, essentially, that he was mistaken or deluded. But you were polite, so no real problem, but when he insisted, and then someone else cited the physics of why he could be right, then I seconded that and cited ample evidence, you persisted in preaching like you were lecturing a bunch of 12 year old tenderfoots who insisted there ARE such things as snipes.
Maybe you've spent too much time with kids and now treat everyone like kids. That's one reason I generally don't like the company of K-4 teachers.
To put it as bluntly as I can, you're not talking to a bunch of wide eyed 12 year olds learning to swim in between summer camp troop initiation haizing. Your audience here are adult divers, who are at least literate enough to use a computer. They had to understand the principles of buoyancy and displacement to complete the first module of the easiest path to becoming divers. Many of them had to swim the length of the pool underwater, so they can hold their breath a decent length of time. They're probably pretty comfortable and relaxed in the water. So, even though I know everything you're saying about floating, before you say it, when an ADULT CERTIFIED DIVER, a member of a sub-population where the most common buoyancy complaint is that they can't sink without enough lead to build three car batteries, tells me they can't float, I don't feel any impulse to condescend to them about mindset and really, really trying. I grant them the dignity of not treating them as either a liar or a fool. Furthermore, when multiple certified divers are saying yes, there are people who can't float, I don't persist in insulting ALL their intelligence by repeating the same mantra over and over again, in the face of overwhelming evidence that yes, they DO get it, and they DO know what they're talking about.
Yes, I freely admit, condescending is one of my hot buttons, but even now, you persist in taking a stance and tone of "Yeah, some people can't float, but the ones I'm talking to now couldn't possibly be among them, they just need to be told ONE MORE TIME about the mindset thing and they'll come around. Poor dears."