A good point t.d. #179, however perhaps a legal interpretation might judge otherwise.
My company’s policy required an understanding of all commercially rated add-ons, this was recorded and the technical and operational judgements were shared with operators and regulators. This provided a basis for a defence, with a fall back of proving that the change was not known of.
In the ‘Executive’ market where there were many more enhancing ‘gismos’, the policy was even more ridged.
The add-on had to be formally approved to the standards of part 25 - technical equivalence, usually requiring a company evaluation / fight test; or if not, then the modification centre / operator were informed of the reasons why a device would not be approved or the outstanding risk which they held; i.e. preventing part 23 or lesser quality ‘STCs’.
Many of today's ‘add-ons’ apply to a wide range of aircraft types and operations. System vendors often cover this with let-outs; ‘not for primary use’, or ‘advisory only’ (how can enhancing awareness be advisory - having to ignore something already known).
This leaves a safety gap to be closed by regulators, yet even they duck the issue with ‘the operator must show, etc.’
Another complication is that the originating aircraft type certificate (country) is often transferred to another county's regulatory system without amendment or knowledge of what responsibilities that carries - i.e. questioning add-ons. Similarly with operational approvals, usually based on vendor material (insufficient thought about the ‘get outs’), or again transferring the judgement to the operator.
The primary manufacturer exports a design /operational culture - the aircraft and training, the system vendor similarly, but not necessarily identical technical or operational cultures.
A third party applies his version of these cultural aspects in operations; any disparity enabling opportunity for error.
Not an A vs B point, but compare cultural views amongst designs, operations, training and support.
The crux of your point is who approved the system interface and operating procedures, but I suspect that will not be the key issue in law.