Would mean that it is the すし that does the action of 食べる. So for this sentence you cannot use が.
As your textbook says, it means "I ate sushi. (Literally: As for sushi, I ate it.)" and it gives a nuance of contrast: "As for the sushi, I ate it (, but I did not eat the other food)".
Is the most neutral way to say "I ate sushi".
If you get asked the question 何を食べましたか。 (what did you eat?), however, you can only reply by using を (すしを食べました) and not は. The reason for this is that the question is neutral, so answering it with a sentence that shows a nuance of contrast such as すしは食べた。 would make it quite unnatural.
That being said, if you get asked the question すしを食べましたか。 (did you eat sushi?). You could reply by using は, and say something like すしは食べませんでした。