For a CPU vendor, ISA is very relevant: most buyers will start their buying decision with ISA choice already fixed, and a vendor who can't offer a CPU with that ISA simply isn't in the race.
It does not matter whether you are a believer in horses for courses when it comes to ISA, or a believer in "frontend ISA does not matter because it's all translated away anyways": when buyers don't want what you have, you are out. And buyers are more like a stampeding herd than like rational actors when it comes to ISA choice. I'd see offering CPU for multiple ISA as an important hedge against the herd changing direction.
What AMD wants to achieve with their CPU: sell them, preferably at a nice profit. If ISA is truly not relevant for performance and efficiency characteristics, all the more reason for them to not bet on any particular ISA but spread out, to already be there wherever the buying herd goes.
