The footnote and the entry was a bit rushed and was not intended to be snarky.
To the best of my knowledge - please correct me if I'm wrong - you cannot say a secret should be valid in a time period in Vault. The TTL will tell the client that they should check back with Vault for the most up to date secret when the TTL expire. If the client credentials are still valid, and the secret has not been revoked or updated in some other fashion, the client will fetch the same value again. Hence the claim that it does not support secret value expiration.
To the best of my knowledge - please correct me if I'm wrong - you cannot say a secret should be valid in a time period in Vault. The TTL will tell the client that they should check back with Vault for the most up to date secret when the TTL expire. If the client credentials are still valid, and the secret has not been revoked or updated in some other fashion, the client will fetch the same value again. Hence the claim that it does not support secret value expiration.