They claim PCIe4.0 makes it irrelevant, but that doesn't really make sense and it's most likely the case they want to charge a fortune for their high-memory options.

It is if you can pool the memory. It's easier than having to do the split of the models in software (though that's a somewhat solved problem) and from what I know allows higher GPU utilization on both cards when they don't have to wait for information to pass back and forth.

it is barely relevant to big players, but is extremely valuable for small players: distributing your workflow to multiple gpus manually is not that simple thing to do and there are a lot of much more interesting/important problem to solve than shoving your model to a gpu.