And say they did use 10% of all GCP? Would it be less impressive? This is a result that was considered by experts to be far beyond the state of the art; it's absolutely ok if it's not very efficient yet.

Also, for what it's worth, I'm pretty sure that I wouldn't have been able to solve it myself in three days, even if I had access to all of GCP, Azure and AWS (except if I could mine crypto to then pay actual IMO-level mathematicians to solve it for me).