As of DPL 4.1, the tool bars are greyed out if defaults are set to NoCorrection and if folders contain TIFF files (have not tested for any other formats)
The tools work and the titles illuminate when a tool is active. The passive state makes the tools hard to read though, see screenshot (upper: DPL4, lower: DPL3)
I’d rather not like DxO to follow Apple’s current “diluted grey on grey fine print” gui design rule that has creeped up to its current climax in Big Sur 