Clearly the common way is more efficient than the original suggestion, given that people achieve useful fluency in a language much quicker than spending 40 hours a week for 7 years listening to it, and only then starting to consider speaking in it.
I can only assume that the OP misspoke when they said 15,000 hours. In reality, when and what you output is dependent on your “input fluency”, as opposed to some arbitrary number of hours. In other words fluency is not an on/off switch. You will develop pockets of fluency as you acquire the language, and those pockets become suitable for output.
Sure. But if their point is not "consume a gigantic amount of stuff passively before you consider speaking", I'm not sure what the difference to the conventional way is, given that "immerse yourself in the language, consume lots of native media, ..." is extremely common and widely recognized advice.
