WD had already been a RISC-V foundation member before their involvement with Esperanto Technologies (who has also been a member for a while). I suspect they saw Esperanto's portfolio and team after meeting at one of the workshops, and bought into it because of preexisting interest in RISC-V.
Just to be clear, I think they bought into Esperanto, but I don't think they acquired it. Much public communication implies that Esperanto Technologies is still generally autonomous[0][1].
