From the European parliament:
Until 1 January 2015 organs were removed from executed prisoners in China without their consent and transplanted. "Although the situation has improved slightly, the secrecy over death penalty figures and organ transplant statistics are made it so far very difficult to gain an accurate picture", said Jöelle Hivonnet from the European External Action Service.
Hard to find any hard and fast analyses here, particularly not from reputable sources. (Some of the key sources for the Medium article are campaigning Conservative MPs trying to win anti-Communist points.) Seems like China might very well be selling the organs of executed prisoners, but it's very hard to prove, and that still isn't exactly the same as "harvesting organs in concentration camps."
If it is true, then it's insane that MSM isn't talking about it 24/7.
Occam's Razor. More likely that this wildly flagrant and proven human rights violation is being totally ignored by MSM, or that the claims of this Medium article, supported by a few British tabloid pieces, are exaggerated?