I eventually got an answer from the support. Hard to believe but well, we’re used to UI inconsistency in Photolab…
The observed behavior is intentional. The first Denoising & Demosaicing section is for RAW images. the second is for RGB images and the third is for non calibrated RGB images.
Nobody had the simple idea of making this clear in the documentation or even in the help popup that appears when clicking on the question mark of each section.
They admit that it’s confusing (understatement). This will be fixed in a future version.