IMHO, he did say you'll be passable as a junior dev(depends on the person's potential i guess). I think that this would be enough for most companies, especially if they think you have great potential and can grow/learn exponentially(like be a senior in 6 or less months)
There are some projects that don't need senior devs THAT much, like basic CRUD apps, etc. And you'll be "continuing" your studies under the company's mentors/senior devs.
So yes, I think 8 weeks of studying RoR can make someone useful to a company. Not all, but probably one that needs juniors