Michael Hampton comments were the solution for this.

It turns out that OMV in fact didn't use ZFS at all but just marked the drives as ZFS members.

I have dd-ed one of the drives and on the image I ran testdisk. It turned out that there was a 0x0700 partition on the disk. Wrote a new partition table with testdisk and mounted it in loop. It turned out to be an ext4 partition with corrupted journal. After fixing the errors I was able to salvage all the data. Hence I did the same on the physical disks, got the data back.