Cephalopod and decapod brains aren't much like human brains either, yet it's still reasonable to regard them as being sentient and worthy of protection from unnecessary suffering.

I'd argue back that using the substrate as a reason why there should not be consciousness seems quite weak. A competing thesis is that what matters is the emergent properties, whatever the support is. So far it has been true for some really high-level tasks - writing coherent text, programing, following instructions, analyzing images... I do not see why the substrate argument would work specifically for consciousness - i.e I would believe it only if I see strong evidence of it.