Is flat frequency response a hoax?

I have seen a number of speaker companies claiming that their design has a ruler flat response. They then proudly display a graph with a flat line as if it's an indication of its superior quality. Poppycock.

Flat response is a myth. Everybody has different hearing so there's no reason to expect that there would be a one size fits all frequency response.
We don't all wear the same sized shoes do we? Everybody has different sized feet. 

There are plenty of flat response speakers that sound horrible. I've heard quite a lot. There are many non ruler flat great sounding speakers too.
Do not be duped by this hogwash. 

If the mixing engineer used a pair of speakers that is 3db down in the mids and top end then he may have boosted that region by a similar amount. Now when the track gets played on a ruler flat speaker it sounds too bright. So as you can see, a flat response is useless.

but sound very different because of phase shifts (different harmonics summing). Still, all measurements are useful.
poppycock. if different harmonics are summing they will show up on the frequency response. 
And, golden ears strikes again...

Have you let the adults in the room know that you are playing on their computer?

If they ask what you're doing, just tell them you are trying to inform a highly knowledgable community the correct way to think about speakers, and, how they have been wrong in their pursuits through the entirety of their decades long journey. 

Remember, a hobby is supposed to be a journey, not a destination. 

You only care about the destination, apparently one that you, and only you seem to be able to fully appreciate.

Betcha those earbuds go mighty loud, huh?

poppycock. if different harmonics are summing they will show up on the frequency response.

Not true - they won't.  Frequency response shows only amplitude (loudness) at each individual frequency and not how combination of them like chord or tone with a lot of harmonics will sound when there is a phase shift between them.

People are actually responding seriously to this. I love it. This is the funniest thing I’ve read in days. 
Your hypothetical mastering engineer, who masters on a single flawed speaker and, what, hasn’t even bothered to zero out the frequency response before analysis, is ludicrous. This hypothetical person/situation is, as you say, “poppycock”.

A circular, self-referential straw man argument—precisely like the one kenjit set up with this caricature of a completely unprofessional mastering engineer—is a worthless rhetorical device. If your goal is to arrive at truth, this is a disingenuous way of going about it.

In reality, a professional mastering engineer reviews program material across numerous grades and styles of speakers; from mobile devices to planar magnetic/electrostatic, to massive vintage JBLs or Altecs, and even gigantic audiophile multiway towers like Dynaudios and Wilsons.

The situation kenjit posits, a poorly mastered recording made by an unprofessional engineer, is useless because such a poor recording will only sound correct on a system with the same flaws as the original one that the recording was mastered on.

This is precisely why a professional mastering engineer first zeros a mastering system to a reference standard before going to work. It is also why this entire thread is pointless.