It's cartoonish, like much of what he wrote.
To be as charitable as possible to Wiernik, he would have to be referring either to the corpses he claimed to have seen on arrival at Treblinka ("The camp yard was littered with corpses") or some that arrived shortly thereafter by train ("Late in the afternoon another train arrived from Miedzyrzecz (Mezrich), but 80 per cent of its human cargo consisted of corpses.") This allows that they may have already been dead for a couple days. The other four groups of people or bodies he described by this point were all selected and killed after he arrived in camp so could not be decomposed.
Wiernik says he carried some corpses "out of the train" on his first day, then on the second day he was assigned to drag corpses "to their graves". This continued for "four days". So at the latest this could only be a description of his fifth day in camp.
Could corpses begin to decompose over five to seven days in August weather? That is possible. There is plenty of literature about this. They could smell bad and have maggots. Their skin could become degloved when handled by humans, a detail that Wiernik did not mention. But the literature has very little to say about limbs becoming detached in handling.
This article in the
Encyclopedia of Forensic and Legal Medicine twice mentions that when moving bodies their limbs can detach, first in the context of "significant decomposition" and second in a body that is "severely decomposed".
Unfortunately there are no numbers to go along with this, but it is difficult to imagine how bodies could reach a state of "severe" decomposition in the short timeframes described. It is also against common sense. Decay naturally starts at the outside of the body and in wet organs. Limb joints should not be one of the first things to decay.
I also can't see any reason why a pile of "mangled bodies" would be left in open view of the train station, as Wiernik described. Nor why the killing methods would be so diverse: blunt weapons, machineguns, and gassing, for different groups at the same time.