«لَكِنْ أَصْلَ إِنْ عَدَمُ الْجَزْمِ بِوُقُوعِ الشَّرْطِ»
The basic function of in is to avoid definitively presenting the condition as certain to occur.
This is not grammatical jazm, whereby a particle governs a verb’s mood. Here jazm means a speaker’s commitment to the condition’s actual occurrence. Conflating the two senses of the same technical word produces an error.
إِنْ جَاءَ زَيْدٌ أَكْرَمْتُهُ
If Zayd comes, I shall honor him.
The arrival is posited as a condition without being asserted as an event that will definitely take place.