It's not as far-fetched as it sounds. Audio mastering has in fact been done this way pretty much since its inception. So its really nothing new.
In the olden days, audio engineers would often use terrible-sounding speakers to better approximate the sound of car radios. 45's were mastered to sound good on jukeboxes, or affordable home systems (which were often monophonic, and far from a flat response by today's standards). Because that's where people listened... So from a consumption standpoint, it made perfect sense to do things that way.
The quest for a "flatter sound" didn't really begin in earnest, until maybe sometime around the 1970's, when some early audiophiles began making larger investments in home audio gear. And equalizers (the type that would sit in a stack with your amp and tuner) became more of a staple in home audio setups.
This is why whenever questions come up about the "ideal sound" for a pair headphones, I will often try to make the point that the intended listening space for the recording is often just as important (if not moreso) as anything else.
Audio engineers will master the content to sound good on whatever people are actually listening to. Because that's what they're paid to do!
There is an upside to this equation though... And that upside is that the headphones, IEMs, home theaters and "radios" that people listen to in their cars and home offices (and even mom's kitchen) are getting better and flatter sounding overall. The audio engineers also know this. And should also begin to adapt their mixing and mastering to take this into account.
Ideally, you want your recordings to sound halfway decent on a variety of different gear and transducers. And the only way to really achieve that with any sort of reliability is to master the content so it sounds halfway decent on headphones, speakers, IEMs, etc that are close to a fairly neutral "flat" sound.
If you're mastering content for a specific audience though, then you'd be foolish not to check how it sounds on the equipment favored by those listeners.