This might be a whole 'nother topic unto itself, but physically, yes.
Let’s get a bit of related background out of the way first: I know there are some users here that are also into making music, and for all of you that don’t, there’s a truism in mixing that says that if you make your mix sound good in mono, it will sound good in stereo. Addendum to this, if you make your mix sound good on a cheap speaker with little to no bass, it will translate really well on any system.
Now subliminal tracks are not music per se, where instrument placement and mono compatibility play a major role - but here’s one thing that mixing on a crappy, single-midrange-driver mono speaker has taught me:
It’s about the single membrane.
If you mix on a speaker that only does bass-midrange, or mid-high, with no tweeters, no ported bass, no need to monitor crossover frequencies, then at any moment, the driver translates information into air pressure that drives one single membrane.
So if you have a bass hit, a cymbal, a piano, a strings section, a vocal all playing at the same time and you take a tiny snapshot of what’s actually coming out of that speaker at any given point in time, something will have to give. One membrane cannot accurately reproduce all of this frequency information all at the same time. It’s one reason why bass is notoriously tricky to get ‘right’ in a mix, because it eats away so much headroom for the information higher up in the frequencies: strong bass is what makes the membrane wobble like crazy - try stuffing extra information into something in such an excited state!
(That’s why you want to sidechain the bass to the initial transient of the bassdrum, snare drum, etc, which is a digression. We really shouldn’t sidechain subs to our music.)
OK, recap: you can only put so much information through one single membrane at any point in time. I believe, and I’ll be happy to be corrected, that when the manual says that “it is OK to listen to music or watch videos while also listening to subs”, what is being meant is - “it’s OK to have one set of speakers play back the subs at a volume that is comfortable to you, while also watching TV and listening to its soundtrack over the TV speakers or a secondary Hi-Fi system that the TV is hooked up to.”
Extending this thought, apply the physical-one-membrane-model to listening to subs on your phone speaker. Imagine the tiny driver and take into account the seriously limited frequency response of that thing, crammed into a crowded chassis. (I’m seeing a lot of density of information in the low-mid range of a typical, masked SC sub, which a phone driver is optimized for - think human voice, it is a phone after all 
) Now imagine playing back music over this overworked little driver at the same time that you are playing back the sub.
(This is why cheap earbuds are not recommended - they cannot translate the information accurately with tiny drivers mass-manufactured for the biggest possible profit margin on a $5 item.)
Now, let’s consider the other single-membrane player in this equation: your ear drum.
You can follow a conversation, have “half an ear” on the TV and be quite aware of your favourite song playing on the stereo all at the same time; your ear is actually quite fantastic at high-res.
(Consider that you can fool your eyes into seeing smooth motion with only 25 frames per second, but how incredibly more sensitive your ears are at detecting skew and flutter, and how even 44.100 samples per second are not high-resolution enough to fool some trained ears 
)
Plus, there’s an undeniable upside of distracting your conscious mind with something pleasant like your favourite audible playlist, while slipping the subs in through the back door at the same time.
Just, whenever you think about playing subs back over less-than-stellar delivery vectors: keep the single membrane idea in the back of your mind, and you’ll make a useful decision
(Not actually directed at you personally, @Introvert_Uli06 - it’s been a post that’s been burning on my fingertips for the longest time.)
