Not OP, but
> This is false. Please stop writing false statements without any links. NXP promises to produce the i.MX 8M Quad until Jan. 2033. The support will be even longer.
I think they meant that the processor itself is old. It supports ARMv8 and is lacking the enhanced memory protection and execution features of the ARMv9-A processors on newer phones.
> This is false again. It doesn't matter how much my device might be compromised. The attacker will not get any access to the shut down sensors, radios or voice/video, if I use the three kill switches.
The problem is that your device can be compromised quite easily and without you knowing. The kill switches are moot at that point.
The kill switches will work independently on a compromise. Why are they moot? Also, it's possible to completely reflash the device in case of doubt.
"quite easily" strongly depends on what exactly you are doing. For example, if I use Firefox with NoScript, then it is not very easy.