If I remember correctly, this salary was offered to Andrej Karpathy, who's easily the top 10 in the world in DL field. So, no, it's not a bubble. Top experts have always been paid well.
DL is over hyped. This idea that people from a few AI labs are producing everything is basically SF VCs sniffing the farts of AI departments near SF telling them that only their labs and the labs of a few friends are magically producing pixie dust. It must be nice to be so well connected. When the bubble pops very soon, 1 year on a 500k salary will have been enough for them anyway.