Incorrect lightDir vector in lesson 5.3 Shadow Mapping in fragment shader

[Vakhtang35]

We're modelling a directional light source and all lightDir vectors must be parallel to fixed one. So why we're calculating lightDir vector as (lightPos - FragPos) like for spot or point light source?
https://learnopengl.com/code_viewer_gh.php?code=src/5.advanced_lighting/3.1.3.shadow_mapping/3.1.3.shadow_mapping.fs