As a product manager I wish I had spent more time on either learning to program or preferably designing interfaces for product ideas that I had. I did some of this stuff but not as much as I now realise I could have.
As a CMO the best evidence of your skills is in marketing a product of any kind. It could be a blog or product that you've created yourself, or pro bono work for someone else.
None of this is to downplay the chops of this artist - it really is amazing.
Sure, I can start a blog talking about social media marketing, but basically it's starting from absolute zero - whereas Noah even early at his earliest started from a place a bit higher. For me to start a blog and say unique things about marketing that haven't been said before and then get that noticed... I consider myself to be smart, but I don't know if I'm on that level.
And sure, like you and I both said, I can build a product (I'm trying as we speak like everyone else is on HN) to develop my marketing skills, but then lets face it... if it's actually worth doing, my job isn't going to be primarily about marketing; it will be about leading a team, building a great product, and yes a bit of marketing.
As you know, marketing is all about the packaging, and how you sell the sizzle, not the steak. So even if you think it's all been said before, it doesn't have to stop you from saying it in your own way, with your own unique insights, distribution methods, etc.
Like Noah you can find an old master to train under too.
A lot of self-taught coders do this by building websites for friends and family when they get started, why not do the same with marketing?
But overall agreed - not easy to demonstrate marketing chops until you're in the hot seat. :)