> why would they be running such an old Linux?

They didn't.

OP misunderstood what gVisor is, and thought gVisor's uname() return [1] was from the actual kernel. It's not. That's the whole point of gVisor. You don't get to talk to the real kernel.

[1] https://github.com/google/gvisor/blob/c68fb3199281d6f8fe02c7...