In my opinion, I think the most important skill needed to be a great product marketer is sales (specifically pitching). You have to be able to get in front of a customer and pitch your product well. This means understanding your customer's needs, explaining your value proposition in the first 90 seconds, succinctly showing how your product can solve your customer's problems, and why your product is better than other products that s/he is considering.
My ex-boss (CEO of Leadspace and former CMO of Salesforce) told me that 75% of my value as a product marketer comes from creating the perfect pitch. I know the market, I understand customer problems, I know the product, I see the competition, now I come up with a pitch that resonates with his customer base. That's why he's paying me.
If your pitch is not good, then the rest of the marketing collateral you create won't be effective. Perfect the pitch, then replicate it throughout your marketing and sales materials.
After you have your pitch down, then you can work on the other product marketing skills such as scaling your knowledge and providing strategic guidance for your business.
I explain these skills more in this blog post: https://www.leadspace.com/what-makes-great-product-marketer/