What “approach”? That thread just mentions that he is insisting on using (imho) inappropriate cabling for his installation, recognizing that the under-sized wires will result in significant voltage drop due to distance, and wondering if it would work. The bulk of the discussion is about a mismatch between theory and practice w.r.t. voltage drop calculations.
And what is “really ethernet cables”? “Ethernet” is just the usage, not the specification. It really has little to do with the wires themselves, but rather the sheathing and physical properties associated with the cable as a whole.
Do you have a REASON for wanting to use non-standard cabling? You really haven’t given enough information to provide advice. You hinted originally, since you asked about waterproofing, that it is an outdoor installation (although perhaps you’re just concerned about potential leaks at the valve – which is certainly something to be concerned about!). Using indoor rated cable in an outdoor situation – especially buried – is asking for trouble.
If you just want to know if it is electrically possible… Ray gave you the answer. If you want to know if it is a good idea… give us more details or accept my answer and go buy the right cable for the job.