Yeah, there are plenty of people who eat mostly grains but are nevertheless perfectly slim and healthy. In fact, I was one of those people a long time ago. There's a mountain of common-sense counter-evidence against the claim that eating a lot of grain necessarily makes you fat. That's what those people are bringing up.
So what's the problem? You tell them that grains are fattening because they promote fat storage or whatever? And then they counter that they know plenty of people who are slim and healthy "despite" eating a lot of grain? Well, they have a point I guess.
Or do they? For somebody overweight or obese, making a no-grains rule and slimming down as a result is a completely different animal than somebody already slim and healthy making that rule. The no-grains rule doesn't necessarily apply to somebody who's clearly doing fine on grains... that is if we're only talking about losing weight. If you don't need to lose weight, of course you don't need any rules for doing so.
You say to your overweight friend, "Hey, why not quit grains and see whether you lose weight?" And then they respond, "But but BUT... there are plenty of grain-eaters who are healthy and slim!" Well, who cares? Yes, there are plenty of grain-eaters like that, BUT THEY AREN'T ONE OF THEM! It's not a hard-and-fast rule that eating grains makes you fat, and I don't think anybody suggested that anyway. All we know is that that rule has been extremely helpful for losing weight for many people.
Either way, I wouldn't be surprised if the no-grain rule only works by accident. If you advise somebody fat to stop eating grains, there are thousands-upon-thousands thousands upon thousands of crap products that are now off-limits. Most processed food includes grains in at least some sort of form. And the worst industrial imitations do too: Industrial pastries, cake, pie, pizza, etc. Quitting grains means eating less processed food and more meat, fish, dairy, veggies, fruit, etc. No wonder it works!
This is the same reason why the raw food diet works wonders for many people. If you make it a rule to never eat anything cooked, what junk food or industrial whatever do you have left on the menu? Nothing really. All you got is fruit, veggies, nuts, seeds, etc. Somebody switching from the SAD to that is gonna make a miraculous transition, even if it might be very suboptimal in comparison to a paleo or traditional eating routine.
See what I'm saying? Stare those people in the face and ask whether they're preparing traditional rice, pasta, bread, or whatever from scratch and along with other traditional ingredients, or if their grain-happy diet is a mix of bread from some grocery store, cake on their friends' and family's birthday parties, pizza from the local food court, pastries in individual plastic wraps from the local convenience store, etc. Maybe... just MAYBE... the people who do well on grains are those who eat REAL grains, avoid the junk food to a greater extent than normal people, and perhaps exercise every once in a while.
But then you would have to change your whole argument, wouldn't you? Why exactly put so much faith in this grain-are-evil philosophy anyway, if it seems like some random dude you know is poking holes in it in a casual bar-room conversation? The fact is this there are plenty of people who do fine on grains, but those are the people who eat REAL bread, REAL rice, REAL pasta, etc... not the people who stuff their faces with individually-wrapped pastries and fake-tasting cake.
If this rule really does work by accident in that way, that explains why the no-grains rule works so beautifully for fat people even though you can easily point to many people who do fine on grains and aren't fat or anything. The ones who are doing badly are the ones who need to stop eating all that industrial crap, which the no-grains rule usually takes out with a glancing shot. And those who are already doing well even though they eat grains already avoid that industrial crap pretty well. Get a SAD-eater to go raw, and there will be a revolution. Get me to do the same, and I'll do worse.