They cannot ignore a complaint.
Do you have a source for this, because in my experience private companies can and will ignore complaints on a regular basis. Either by literally ignoring it, or by sending out a generic form letter along the lines of 'Thank you for your complaint. After review it has not been upheld. This is not an appealable decision'.
You've been quoted the "source" for this above. If they do not follow their ATA CoP and respond to a complaint then they run the risk of being suspended or worse which would mean that they no longer have KADOE contract with the DVLA and cannot simply click a button and receive keeper details which would severely affect their business model.
However, even if they have to treat the complaint as an appeal, they still have to respond. In my experience of dealing with these issues over many years, NCP are one of the few companies that regularly either don't use PoFA or mess it up, such as sending PoFA NtKs but outside of the required timeframe. In my experience, they have never tried to pursue a keeper who has highlighted their PoFA failure.
So, in this instance, if the OP has sent the complaint as advised, I believe that they are likely to cancel the PCN. We won't know until the OP tells us the outcome.
As for the minutiae being argued about how punctilious comms need to be, at least at this stage, is a bit academic.