At most of the companies I have worked for, the difficult part of my job is taking the pre-existing code that barely works under the old internal proprietary framework and rehabilitating it into the new internal proprietary framework with a bare minimum of new development.

Showing off that I know how to write (or recognize and copy) good code from scratch does not signal for that. I can't tell you how many times I have had to code something that is objectively incorrect because the old version did it that way, and the customer doesn't want the new version to be different.

The problem with the whiteboard interview is that you can't show an old, crufty code base on it, and ask the candidate to sketch out how to implement a simple feature request without overhauling the whole thing to use industry best practices. And you can't show an existing relational database schema that is pathetically ill-designed and ask about a new report for the customer. Those are the sorts of problems businesses have. They don't suck it up and pay the big bucks for a software pro until after the amateurs have already tried fixing it with an Excel spreadsheet or an Access database. And they don't want you to mess up the solution they already have, no matter how much baling wire and duct tape is holding it together.

Not everyone that can write code can also work well within an existing codebase. The only situation in the wild that I would trust to indicate that skill for my company is someone who actively contributes to an open-source project with a lot of other active contributors. A one-person side project that shows off everything you can do all by yourself won't reassure me. In fact, if it's too good, I might be worried that you will want to overhaul working code just because it's really, really ugly, and you can detect multiple code smells when you look at it.