" Brandon replied that while Hoid himself is not a shard, he is bonded to one" -- actually I recall public statements by Brandon that seem to deny this.
Fair enough. You would know better than I would, certainly.
And since when is hoid immortal?
Brandon said Hoid was there when Adonalsium was Shattered. Also, it's been said that it takes a very long time for a person with a Shard (say, Ruin) is molded really well with its Purpose. I'd venture at least centuries, more likely millennia. For all practical purposes, he's immortal.
And, well Brandon said in...an interview?...a QandA session?...(I'm not great about citing sources, apparently) that Mistborn happened first, with Warbreaker and Elantris some centuries after that. So, immortality after a fashion seems the only viable explanation. Or time travel.