I don't think that when people commonly discuss sentience they mean to include goldfish. I don't think the legal definition (which probably exists due to external legal implications) has any bearing on the intellectual debate of AI sentience.
Sentience is just the capacity to experience feelings and sensations. Goldfish can do it and AI can’t (so far).
If I were talking about sentience I would definitely be including goldfish. What about them is so different to us that we would have sentience while they would not?