I worked at Vodafone and all the routers get a security review, so it is highly doubtful that they left it in on purpose because they would have known that it would have been found. It was test engineering firmware that got left on some routers, and makes spurious claims even though in the article it points out that Vodafone didn't believe it was a backdoor.
And your first link talks about how someone appears to have added an additional chip to the design, which is unlikely to have come from Huawei. Sounds suspiciously similar to this
https://www.bloomberg.com/news/features/2018-10-04/the-big-h...
Do you want a list of everything that Cisco (replace with your preferred networking manufacturer) messed up and claim that it's all malicious?
https://www.theguardian.com/books/2014/may/12/glenn-greenwal...
Tampering with these things is allegedly pretty simple.
https://arstechnica.com/information-technology/2019/10/plant...