Yeah, pretty sure there's no such thing as solid 6 AWG. I'd expect that to be somewhat fragile and dangerous.
Interesting that the 80A Gen 2 Wall Connector advises ferrules, they surely must be legal then, at least for this appliance, but I'd still question the legality of using them on a breaker or outlet that's designed for bare wire. I thought I read something in the NEC code stating that ferrules were only legal if factory-installed but maybe that only applies to smaller wires that are more subject to amateur installations.
The 200A stranded mains don't have ferrules in any breaker panel I've seen, nor do the common 50A-100A circuits for subpanels, A/C, Jacuzzi, Dryer, Stove, etc. but perhaps the difference with all of those things is that you can hold the loose breaker/outlet in one hand while inserting the wire with the other. The HPWC requires you to fish the wire blindly into a dark hole and just hope it's in right. Maybe that's why they suggest ferrules?