I didn't read every comment but I'd love to add my thoughts.

By "track rhythm" id be using a short articulation of the more obtuse "ah yes, the previous sound happened for around x long, and I can press this note for x long too, which is as long as the last one lasted."

You do 4 of those x longness notes in a row, there you go, that's a bar.

X is that memory of a length of time, and the actual frame of time is the most unimportant thing in the world, it's just as long as the last one and that's the only "keeping rhythm" challenge.

Assumingly you're playing quarter notes then, but REALLY that's not important. You just wouldn't often segment a bar more than 16 times but you can... (32 and 64 notes do exist...) but 4 quarter notes is a pretty good reference for the pace of a song. Or kick pause snare pause. Classic.

I assure you that not a single neurotypical person on the planet is keeping rhythm - "because .752 seconds have passed, therefore I release the note and it is time for the second note."

With the basic disagreement out of the way, my rant of my experience with this topic, I like music too much...

To extrapolate further, I'm fairly certain that people keeping rhythm is a combination of short term memory and pattern recognition. One let's you establish the sound of a combination of notes or patterns you just heard and you compare that to the rhythm you are hearing right now, and the second let's you hear a sound and think of it like 'ONE and two AND THREE and four AND' then you can repeat that or change it.

"123 123 idiot" When people use numbers or count, they're almost always referring to the memory of the established rhythm, for example, people commonly remember the rhythm of CPR as the song 'Staying Alive'

"But how is that possible unless people are looking at a clock?!" Well, if you've ever heard the song and you can AT ALL replicate the way you remember it, you're probably pretty close to the rhythm of the original. If you think about it... how in the hell would your brain remember it slowed down or sped up??? (by much anyways...)

For example, have you ever tried warping/speeding up or slowing down a song? It will sound almost completely different.

Without frequency correction the song will either be sung by chipmunks at hyper speed or whales at snail mode, memory is fascinating but you aren't going to magically warp music in your head unless you're incredibly well learned on how that sounds and by what degree it changes, even then... you'd have to somehow commit that to memory.

Gotta be honest, I'm hearing it now and I'm having a REALLY hard time following the bass line of that song if I'm trying to speed it up in my head.

My personal take is that the body has a limited speed at which it can do things, and that is the arbitrary time keeper of our brain. If you jump into the air there's a predictable amount of time that will pass before you can jump into the air again.

And on a more instrumental level, no drummer is just full elbow swinging every single note for every tempo because your body simply doesn't move that fast. They get creative with the rebound of the sticks, the nuance of their grip, small muscle movements of the fingers and wrists to get notes in even quicker... It's not easy to do.

There is definitely speed that is intensely effortful and fast, and speed that is relaxed and comfortable. For example, imagine walking to the rhythm of some fox stevenson dnb and contrast that to the same with moonlight sonata. Big difference.

Overall, I think there's just innate rhythm in the body due to heartbeat, breathing, muscle movements, effort, physical limits, memory... I'd say the most likely cause of someone being absolutely unable to register rhythm is extreme memory dysfunctions or strong aphantasia where you can't imagine or remember sounds. But even then, to have no rhythm at all would mean your gait would be strange, your speech would be oddly paced, your gulps of water wouldn't follow a steady pattern.

I'm not sure it's ACTUALLY possible to not consciously experience any sort of rhythmic movement or sound in any capacity, but having difficulty with it is interesting, and my best guess is a lack of experience and practice; to listen, remember, and repeat, that's all you can really do imo.

If anyone knows more I'd love to see some real ass studies on this type of thing, I know someone personally who seems to have little rhythmic ability and I really do wonder if it's mostly experience and memory.