If i remember correctly, not all Iron Warriors were at Terra, only a contigent. Most of them held their own little Empire around Olympia, while others guarded the forticiations on numerous other planets. But thats not the point.
They, like the Night Lords, were allies of Horus and fought for his side, granted, but that doesn't necessarily means that they were as devoted to Chaos as, say the Word Bearers. Likewise, not every guy who fought in the crusades was a devoted Christian.
Furthermore it is explicity stated in the IA article that they honor the Chaos gods as a pantheon but are not truly devoted to it. Their loyality belongs more to Pertuarbo, who is a demon prince but not because of his worshipping of chaos (like for instance, Lorgar) but because he nearly destroyed the Imperial Fists.
What i said about the mutations comes also straight from the Index Astares.
As for the Night Lords, they don't ally themselves with particular Chaos Power for a long time and their geneseed is the purest of all the Traitor Legions. They see faith in Chaos (or any other thing except wealth and power itself) as foolishness.
So, of course those two are Chaos Space Marines, but not as devoted to it as the other Legions.