Hello: I would jumper between hot and cold with #6 solid copper wire. The stainless end at the tank doesn’t seem to have dielectric built into it, but you could test this with a multimeter in the ohms times one setting, while measuring from the corrugated line down to the nipple coming from the tank. If you get a reading, there is continuity and you don’t have a good dielectric, (which may not be really important).
I’ve never seen a stainless flex adapt to copper. I’d like to know how they made the connection 🙂 In future, I’d use a ball valve and go to a threaded connection from there so you could use stainless or copper connectors that have unions on both ends.