I’m not sure about that. For example, my NVidia has 6 GB, but can allocate only 1.5 GB in one chunk (‘allows GPU allocations…’). But when used for processing, darktable allocates all available memory, just not in one operation. See
Of course, that integrated GPU may simply be way too slow for diffuse or sharpen.
The ‘preview pipeline’ is different from the one used for export, it only does a partial rendering, and if you are not zoomed to 100%, it does what export does when you run a scaled-down export and you disable ‘high quality resampling’. See