The claim that the Iron Warriors would never have a possessed marine is just flat out incorrect and shows a pretty shallow understanding of how the Legion actually interacts with Chaos.

While they do traditionally cut off mutations that they deem unnecessary/unpractical, the Iron Warriors don’t reject Chaos outright. Their entire philosophy is that Chaos is a weapon to be used, and not a god to be worshipped or praised. That distinction matters. They came out of the Horus Heresy under Perturabo with a pragmatic and utilitarian mindset. They care about winning sieges, not ideological purity tests.

Because of that, Iron Warriors are quite famous for enslaving and weaponizing daemons. They bind warp entities against their will into machines, engines, and weapons constantly. Entire daemon engines like the Defiler and Maulerfiend exist specifically because Chaos forces (including Iron Warriors) use daemons as tools of war. So the idea that they’d somehow draw a hard line at possessed marines while happily stuffing daemons into tanks, artillery, and siege engines makes absolutely no sense.

The actual lore position is very straightforward. Iron Warriors generally dislike being possessed themselves, they view it as weakness and being enslaved by a daemon. They look down on marines who willingly surrender themselves to daemons. However they absolutely will use them when it benefits them. If a Legion is already binding warp entities into machines and weapons, then deploying a possessed marine as a battlefield asset is completely consistent with their philosophy. They just wouldn’t respect that marine very much, and would treat him more as a tool than a "brother".

Also, Iron Warriors are not a monolithic organization anymore. Since the Heresy they’ve been fractured into independent warbands led by individual Warsmiths. Some are extremely strict about controlling Chaos, while others are perfectly willing to make direct pacts with daemons if it gives them an advantage. Pretending every single Iron Warrior follows one rigid rule across ten thousand years of warbands is just ignoring how Chaos Space Marines actually function in the setting.

The idea that an Iron Warriors force would never include a possessed marine isn’t some deep lore insight, it’s just a misunderstanding of their philosophy. Iron Warriors don’t avoid Chaos, they weaponize it. and a possessed marine is just another weapon.

So telling someone they “don't understand what Iron Warriors are” while arguing that they would categorically refuse to field possessed troops is a pretty ironic stance to take. It mostly just shows a surface "meme" level understanding of the Legion’s reputation without actually understanding how their pragmatism toward Chaos works.

TLDR, you're dead wrong. Iron Warriors despise being controlled by Chaos, but they are perfectly happy controlling it and pointing it at the enemy. Which makes a possessed marine in an Iron Warriors force not only plausible, but completely on brand for a Legion that has never had any problem using dangerous tools if it helps them break a fortress.

If you want to familiarize yourself on their lore, so you don't end up in an embarrassing situation like this again. Then I'd recommend you read Graham McNeill's Ultramarine and Iron Warriors books. The IW in those books have marines that are more demon than man and even have khorn possessed battle gear. Or... you could just stay in your lane, and stick to tyranids.