As I understand his argument, it goes
- If you can suffer, you have moral value
- We have an obligation to not cause suffering to beings with moral value
- Some animals can suffer, therefore they have moral value, and we have an obligation to not cause suffering to them
You can then of course ask whether yeast can suffer, which we don't have any evidence of, but you're welcome to stop eating yeast if you feel morally obliged to anyway. Lack of evidence doesn't mean we know they don't suffer. But for the animals where we have convincing evidence that they experience suffering, such as most intelligent mammals, we all have a clear moral obligation to stop causing them harm.
Counting cells doesn't really enter the argument. Evidence of suffering does, which is not just about sentiment.
The inability to draw a perfect distinction between beings that can suffer and those who cannot doesn't stop us from identifying cases clearly on either side of that line.
What Singer eats doesn't really matter for the argument.