It is an assumption based on the fact that the codebase uses cuda and the main backer of the project owns the cluster.

> It is an assumption

Then don't say "certainly"

Why not? If they had bought the compute themselves, it might cost more than $700k.

I am absolutely certain that the compute might have cost more than 100 trillion dollars.

Many amateurs compete in Kaggle, most of them use whatever hardware they have to hand, and a lot of them will use CUDA directly or indirectly.