Yeah I was reading this article yesterday and had a split opinion about it, but ended up mostly agreeing. Mike Monteiro has posted some really great advice in DDS, and clearly has a lot of experience in a variety of areas. However I always love to play devil's advocate, so here are some thoughts:
I think a lot of it depends on some key points - What are the alternative jobs on offer? Is the reader highly self-motivated and reflective? Does the reader want to learn about the startup/business world and be with a product from start, to release, to maintenance and beyond and see all of the nitty gritty details? Or do they want to just work on designing to a brief and not get caught up in all the business stuff?
I think point 6, "Unless you thrive in chaos you will not be comfortable." is really important. If the reader can answer "yes" to that, then a (well run) startup can provide a huge amount of experience in a pretty short amount of time. A lot of it would be learning from mistakes that can be taken to future projects.
However I agree that point 4 is a really strong reason for going with an agency or freelancing, and certainly that route has provided me with so much experience that I can bring with me when we work with startups and on Canvas itself!