The sorts of things they highlighted were: no version control, no code review, production builds happening on arbitrary machines, no automated testing, poor access control on code, no audit trail on code changes, the list goes on, and that's just for the software side. The conclusion was that Huawei were about a decade away from being able to even claim they had no backdoors. And that's a major telecoms hardware provider, trying to sell into governments and major infrastructure projects.
I'm not in the least bit surprised that TP-Link are doing this, and also not at all surprised that when questioned on it they are (so far) unable to actually describe why it's happening or really seem to know anything about it.
I think this sort of product is built in a very different environment to what most HN users would expect.