They're not claiming it's not a backdoor; they're claiming the backdoor is not in "HiSilicon based hardware video encoders", but rather in management software associated with said encoders. The former would be near impossible to fix, whereas the latter can (supposedly) easily be replaced.

Edit: to be clear: 1, I don't know that the backdoor isn't in the hardware itself, but that's what they're claiming. 2, it sounds like the software in question is the manufacturer-provided drivers for the hardware, which is worth emphasizing, but not the same as the hardware itself being compromised.