Attempting RAID 10 using Disk Utility and Terminal. Fails with Error: -69886: Invalid request. Then mounts larger than expected.

I tried to follow both Lifewire Disk Utility steps and SS64 terminal commands.

Each time, tweaking various things, I keep getting Error: -69886: Invalid request

One OWC 4-bay enclosure. Four identical Seagate 8TB drives.

I can create the two mirror pairs either in APFS or JHFS+ just fine.

But I cannot then additionally figure out the happy path successful commands to get the striping of these two pairs for form one overall RAID drive set.

MacOS Ventura. Mac Studio.

One sample failed attempt.

diskutil createRAID stripe OWC8TB JHFS+ disk4 disk7

Started RAID operation

disk4 is already part of a RAID set

Error: -69886: Invalid request

Second sample failed attempt.

diskutil createRAID stripe OWC8TB JHFS+ disk5 disk8

Started RAID operation

disk5 is already part of a RAID set

Error: -69886: Invalid request

As I was typing this I tried ONE more thing. NOT use the 'disk' nomenclature' but instead use the mounted mirror drive name. And voila it worked.

But shouldn't a striped array of TWO 8TB mirrored drives result in still/only 8TB? This shows 16TB mounted. Is it not then in fact a functioning RAID 10 (aka 1+0) drive?

diskutil createRAID stripe OWC8TB JHFS+ owc1 owc2  

Started RAID operation

Unmounting proposed new member disk7

Unmounting proposed new member disk9

Creating a RAID set

Bringing the RAID partitions online

Waiting for the new RAID to spin up "E2FD5FC1-8EE0-4F97-85F6-79D956A00BED"

Initialized /dev/rdisk10 as a 15 TB case-insensitive HFS Plus volume with a 524288k journal

Mounting disk