Zero Knowledge and Computational Indistinguishability

Having some trouble understanding the following line: “Alice conveys zero knowledge to Bob if Bob can sample from a distribution of messages that is computationally indistinguishable from the distribution of messages that Alice would send.”

Could someone give a practical example of this sentence?

Suppose in some protocol Alice is supposed to send an encryption of her special secret $$s$$ under a public key $$pk$$. She is sampling from the distribution of encryptions of $$s$$.
Bob can choose a random plaintext $$r$$ and encrypt it under $$pk$$. He is sampling from the distribution of encryptions of random plaintexts.
Even though Bob doesn't know Alice's special secret $$s$$, these two distributions are computationally indistinguishable (that's the basic definition of security for the encryption scheme). So this message (Alice's encryption of $$s$$) is zero-knowledge.