«هو قول يقال لقائله إنه صادق فيه أو كاذب»
It is a statement about which its speaker may be said to be speaking truthfully or falsely.
This formulation focuses on the speaker's assertion: may the speaker be called truthful or mistaken on the basis of what was said? If someone states “It is raining,” we can check that claim against the weather.
The two formulations point to the same feature. The first describes truth-aptness; the second connects that truth-aptness to the person's assertion.