this exactly. same ppl do for sound, listen in the car, over shity headphones etc. - that's just quality control not the fault of any piece of equipment.

Yes this is universal in pro mixing setups, having filters or even actual physical hardware to provide the sound of stock earbuds, a crappy Bluetooth speaker, sound system in a minivan, etc.

Well, of course it's a good idea to double check with various output methods. But if a mix sounds good on studio monitors with a flattest possible frequency response (preferably even calibrated with an internal DSP) in an acoustically treated room, there's a very high probability it will sound good on almost anything out there. At least that's my experience.