The 5/8 copper tubes it says were for older pre-alpha and are no longer available being superceded by the 3/4 NYLON tube in Alpha Ones.
I would rather have the nylon, no green or corrosion to start a leak. Not sure why you think it's not substantial. If you have a newer home or underground irrigation it's probably plumbed with poly, pex or peb and delivered by PVC with no metal at all.