Currently, the sales channels to be checked on the product UI have a logic of "if none are checked, all of them are available".

Despite this logic being reasonable on the catalog UI, the export Products & SKUs sheet might be misleading - it shows the column with the respective sales channels as "empty"

Either listing all sales channels on this column or, even better, a "all" text if none are tagged, would be a better solution instead.

Tag a product with no sales channels and the attempt a catalog export, checking the column relative to this data.

Knowing beforehand that empty columns might mean that all sales channels were selected.