I find that win rate kind of weird. I accidentally took a game with the little 1 ships, and because they are so difficult to find, we got even and then it was just a matter of pure luck who found those baby ships first. I think I won, but it was a bit annoying and I'll have to be more careful in the future not to take those.
But anyway, being at 84% wins in that situation seems almost impossible, unless the other person is just really, really horrid.
But as to your point, I see nothing wrong with playing just 1 person. I do think it screws with the ELO a bit, because you're being rating on how well you can play one person, rather than how well you overall play the game. I don't think you play a game well unless you can beat various different opponents, proving you can go up against various strategies, etc. But, well, first it's only Battleship.
And second, I don't make that big a deal out of ELO, and lots of other things throw it out of whack, and I don't believe any rating system is without its flaws, so I just don't worry about stuff like that much. It will never work perfectly, nor ever be totally fair. So I don't care to start messing with it or adjust it and make rules about it.
That said, it seems odd that playing mostly 1 other person would put someone so high in the ranks. It doesn't seem like it ought to work like that. Wouldn't you just sort of stagnate somewhere, just playing someone who rarely wins?