Disclaimer: I run a small European marketing agency (though our minimum monthly budget for clients is a few times bigger than discussed & recommended in the article).
I think this is a really good article, and I definitely agree both with the suggestion to use an agency (though I suppose I could be biased here!) - there's a lot of low-hanging fruit and it's definitely possible to do plenty yourself, but unless you really can't afford an agency there's surely more important things to work on yourselves. While hiring dedicated marketing people makes sense when you're a certain size or bigger I've still seen the best arrangement to be as few people internally as possible and being people who are not only good at marketing but more importantly good at managing, and have the bulk of the work handled by an agency. Rather than having to deal in giving whole people specific jobs, an agency can provide small amounts of time as needed from a wide range of experts on different aspects.
I also agree that it's a good idea to be familiar with with it all too, though, because that way you can actually judge which agencies are worth working with and you can actually work with them, rather than leave them alone and hope they're going to do a good job.
This article itself is a really great example of what they explained at the beginning - about writing. It's not a paid ad, but it got me interested in potentially using their analytics product from having previously not heard of them.