Given that the article is titled "Later Stage Advice" and it says things such as switch from not caring at all about marketing to caring about it a little bit once the product is working I think Sam's advice is properly qualified.
But your point is well taken; I've seen it to, but I'm not sure what the solution is. In general I think the advice of staying focused and minimizing burn attempts to address this, but in practice these choices are rarely easy. Sometimes you need to try a bunch of different things, but then you can be saddled with maintenance costs or blowback from customers if you yank the rug out. Also, if something appears to be working and you start to hire to support growing the business in that direction you can get screwed if you hit some ceiling of market appeal.
The cases where a company is so compelling and viral that there is a clear path to scaling are the exception. For every Instagram there are probably 1000 companies that are still considered successful by their founders but at a smaller scale or slower pace. Startups are hard.