Seems incorrect, especially the suffocating part, they have a third lung that can use water afterall, hell we have stories of them hunting sea creatures with and without armour without sinking.
probably just another mistake, different authors saying different things
My knowledge is that the third lung doesn't work forever, it's a stopgap measure that given enough time will stop working, brotherhood it worked for what 12 hrs straight.
There was a thread like this awhile ago, I think it was started by the same piece of fluff, in which someone cited, one of the spec games which stated that Scout armor was light enough to swim in while unmodified SM armor without mods would sink.
Phase 9: This additional lung activates when a Space Marine needs to breathe in low-oxygen or poisoned atmospheres, and even water. The natural lungs are closed off by a sphincter muscle associated with the multi-lung and the implanted organ takes over breathing operations. It has highly efficient toxin dispersal systems.
But not proof.