Thanks for the output.
There are some things to clarify
VM disks on a ZFS storage do not show up in the file system. They are so-called ZVOLs, volumes which provide a block device instead of a file system.
Your pool is set to be non-thin, which is good because you cannot accidentally overprovision it and run into a situation where you don't have enough space anymore, so the one VM disk is reserving the configured size, thus making the pool appear to be used to what it is.
df does not see the ZVOL which is the VMs disk and therefore can only show what ZFS reports back for the file system provided at the mountpoint of the pool, the remaining ~280GB.
I hope this clarifies the situation a bit.
One last recommendation I have to give you: ZFS is a copy on write system, this means in order to function it needs free space available. The rule of thumb is to not use more than 80% if possible, once the usage is at 90% one should really think to expand it in size.
May I ask what kind of VM you are running that needs a single ~24TB disk?