When the nefarious actor is already inside the house, who knows to what lengths they will go to circumvent the protections? External network blocker is more straightforward (packets go in, packets go out), so easier to ensure that there is nothing funny happening.

On Apple devices, first-party applications get to circumvent LittleSnitch-like filtering. Presumably harder to hide this kind of activity on Linux, but then you need to have the expertise to be aware of the gaps. Docker still punches through your firewall configuration.