Low bandwith serial connection? You mean such "low bandwidth" connections like PCIe?Just based on the size and pin out of the chip the story told by Bloomberg is impossible. That chip does not have the needed pin outs to intercept anything between the central processor and the system memory.
If their pictures are to be believed (and I doubt it but lets just assume they are) then the chip is next to the BMC on the motherboard which is used for remote administration. Think AST2500 for example. In which case it could talk to the BMC through serial or another low bandwidth connection including USB etc
And the MR member who said they need separate access to the North- or Southbridge: No, they do not need that, because they are usually integrated in the CPU package.
Such chips can write and read whatever they want. Just like drivers for network cards, graphics cards, ... There is no hardware which stops such attacks. The Thunderbolt attacks of the past years are real. Why not this attack?