This particular whistle blower is not alleging that the NSA or any US controlled entity compromised Ericsson equipment, rather that the US and Danish intelligence agencies collaborated but that collaboration ended up being used against Danish interests.
That’s orthogonal to whether using Huawei equipment is also against Danish interests.
But clearly it's never the equipment that's a problem (who seriously believes this can't be observed or worked around with simple isolation/reverse engineering/relocation of factory/firewalling?) but the elected officials collaborating with the americans.
When your telco buys Huawei, it's not to give an entry door to Chinese security services. It's because it's the best at that price point. If they can explain exactly how Huawei is supposed to backdoor their commercial partners, then fine.
Ironically I have a Huawei phone, without Google services, and after the initial withdrawal phase that was painful, I now feel better knowing Google isn't tracking me and maybe Huawei is. The simple fact is my government would never willingly give them the keys, while they do with little hesitation when the fat burgers come and ask.
Without knowing the specifics. I think one of the issues is that when NSA hooks itself into say a network router, it leaves trace behind (ie. an altered configuration) that can be spotted by the network engineers that maintain that equipment.
I would have guessed they would be a bit more sophisticated than that, perhaps flashing a new "evil" firmware that would look to be perfectly normal and unchanged to those without the correct key.
That’s orthogonal to whether using Huawei equipment is also against Danish interests.