There are a number of possible reasons for this job ad:
Firstly - it's a low-key geopolitical (anti-neoliberal) message implying that Apple isn't keen on Nvidia's acquisition of Arm. Given Nvidia's track record, none of the big tech companies who are reliant on ARM, trust Nvidia to be in control of what looks to be the more dominant ISA in the years ahead. So it's prudent to have a contingency plan for if the acquisition is allowed to go ahead. Also, Apple have enjoyed the freedom to be creative with ARM: perhaps they fear that things will change, going forward.
Secondly, Apple like control. Using RISC-V gives them the ultimate freedom to develop as they might wish, without having to submit their plans to Arm's board, and await an adjudication. The RISC-V license allows companies to retain their intellectual property - so Apple could design proprietary extensions.
Thirdly, RISC-V is a new ISA, and has some well-considered solutions to potential computing issues ahead... Nor is it encumbered (and because of its modularity, it is unlikely so to be) with the scars of changes and work-arounds due to developments in computing needs, as both X86 and ARM have been. Perhaps Apple can see potential to do things more easily with RISC-V than with ARM.
Fourthly, RISC-V is gaining maturity. RISC-V is established in the embedded and microcontroller space (MPUs) and is now beginning to produce application processors (APUs). There would have to be a very good reason for Apple (or anyone else) to continue to pay license fees and royalties if there is a credible alternative.
It might well be that, at least initially, Apple are considering (or are already using) microcontrollers or co-processors that are RISC-V. This advertisement doesn't necessarily mean that Apple are looking to ditch ARM completely. But it's more evidence that tech companies are not going to be beholden to Nvidia-Arm.
Apple has a good record of changing ISAs smoothly - from PowerPC to Intel to ARM... An Open Source ISA might be a safer, better place to transition to.