Considering that they have three important citations of the paper unpublished, I think it's okay to continue to be skeptical that they have achieved their claims. This paper is not complete, and on it's own does not demonstrate that they have solved the problem.
It is always trivial to simulate storing client data that you don't have. So you'd have to invent a system where simulating client data is equally expensive to storing real client data. But, simulating client data can always be done deterministically using a seed, which means you don't actually have to store the data.
So you have to prove that generating client data is equally expensive to storing it, if not more expensive. Given that legitimate client data can literally be all zeros, I don't see how you can achieve this without some overhead.
That overhead will make you less competitive than Sia, which doesn't have the overhead.
I haven't yet gotten to the point where I fully understand proof-of-spacetime, but that is hard when the paper is literally referencing unpublished work. But I'm skeptical they have solved the generation / simulation attack.