Yes. I find it amazing that people running startups apparently have a problem with this concept. How else are junior developers supposed to gain experience and be exposed to hard technical problems, get the benefit of mentorship, etc?
There is nothing wrong at all with less experienced devs working for a startup. It is one of the components for building a successful company. As your junior employees grow, they move into roles of greater responsibility.
I always see startups looking for experienced people as a company started by a couple of non-programmers, desperately seeking someone to make their products for them. It sounds like they'll be worked to death and expected to work long hours while the founder makes fluffy posts on hackernews :P
Seems accurate, though I think his point is that the overall team quality is decreasing instead of increasing. On the flipside, keeping a strong team probably means the team is always much smaller than it could be at any given stage. IMO this is well worth it and I strongly prefer a tiny team. But it could potentially decrease a valuation where company size/number of employees (especially engineers) is taken into account.
It also depends a lot on the company's strategy. If you are building a complex product then a small, highly qualified team makes sense. If your product is basic and requires a lot of customer success, you might want to hire junior devs or non-devs to do basic work (thus growing your team, but saving the senior dev time for more complex tasks).
Therefore making them perfectly qualified for the job you are giving them?