Answer
At that point, this wasn't available for custom commands — the JSON-logic-based Enabled/conditional-visibility configuration mechanism was only wired up for certain standard, built-in buttons, not exposed for custom ExecuteAction command buttons, even though the same configuration syntax is used internally for standard buttons. This was acknowledged as a capability IFS was looking to implement for custom commands going forward — check current release notes for your IFS Cloud version, since this is the kind of gap that tends to get addressed over time rather than remain permanent.