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Except for the fact that many people claim to empathize with invertebrates or even plants. You clearly understand enough biology to think clearly about this topic. All I'm saying is that everyone else should make an attempt to meet that minimum standard of understanding.
Taxonomic classifications are built out around recognizable similarities in animals biological structures. Setting aside how precisely we understand brain function in non-human species, it takes little effort to see that most vertebrates are structured in a way that heavily overlaps with human biology. They are similar. Why would you assume they aren't?
And if you go from a "philosophical" perspective and ask how we could possibly know what they feel, you may well extend that to humans. How do I know you experience qualia?
You're getting so caught up in precision that you're overlooking obvious similarities. Can't see the forest through the trees. That animal has a nervous system and demonstrates a pain response. How much more detail do you need to be able to understand it feels pain?
For creatures without similar structures, we don't make the same assumptions about pain and suffering. Do carrots feel fear? Do mollusks weep? No one thinks about that.