I don't see any reason to doubt them. It's an incredible risk to switch their entire company over to a still-growing ISA. But being able to design and modify any particular core as they see fit without having to talk to lawyers or negotiate a new contract... that's an incredible power.
Were they using ARM? ( Proberly, but may not be in everything they use ) Are they switching over from ARM? Or simply moving their inhouse controller to RISC-V?
The one time license fee were suppose to be a lot cheaper if you are shipping in billions of unit.
Because if they are switching from ARM, I think of it as ARM being lazy and not winning the battle they ought to win, purely from a business perspective.
Not definitive for the whole product line, but at least evidence of one popular WD drive with a Marvell/ARM controller. Google "Western digital Marvell" for more...like https://www.prnewswire.com/news-releases/marvell-achieves-si... (over one billion WD units with Marvell/ARM chips on board)