«وَأَصْلُ إِذَا الْجَزْمُ بِوُقُوعِهِ»
The basic use of idha is to present the condition as expected or assured to occur.
إِذَا طَلَعَتِ الشَّمْسُ ظَهَرَ الضَّوْءُ
When the sun rises, light appears.
The sunrise is treated as a normal, expected event rather than an indeterminate possibility. Idha frames its occurrence as the basis for the consequent statement.
This does not assert that the future condition has already happened. It concerns the speaker’s manner of presenting its future occurrence, not past completion.