I've had two customs built by Alembic. Believe me, whenever I came up with anything that was likely to take my bass outside the normal limits for their basses (23 lb is only plausible for a double neck), they were quick to tell me so. In some cases, where combining certain options together on a particular bass would result in serious performance issues, they would refuse to build the bass. For instance, for years, they refused to build short scale Stanley Clarke basses with five strings, simply because the added neck weight would exacerbate the bass's tendency to neck dive.
Sorry, but having dealt extensively with Alembic, I find a 23 lb bass way too far over to top to be credible without extensive discussion having been initiated by Alembic before they'd build it. Certainly, many have found their heavier basses to be real boat anchors and I've never heard of any Alembic being called "light". But if this bass actually weighs as much as said, there's a lot more to its story. Think of it this way: Why would Alembic be willing to harm its own reputation by failing to treat their customers well? Too much to lose folks, and in my experience, their whole organization knows it.