It's the hadeeth, not the quran.

let's say it's on top of what already exists

okay

so let's change the scenario

quran = core religious document, hadeeth = much longer work chronicling everything the true prophet said in great detail

There are many times more hadeeth than verses in the quran.

Quran, what god said. Hadeeth, what the prophet said and did.

Though the prophet is guided by god.

if I told you there were muslims who read the quran and believed it was a true book, and that muhammad was a true prophet, but regarding the hadeeth they acknowledged its authenticity yet paradoxically disregarded it as "apocrypha" and never referred to it and strongly discouraged its study, what would you make of that?

what would your conclusion be about this hypothetical muslim sect?

There are a lot of false hadeeths. Most hadeeths are false.

And the sources are the same, so it doesn't make sense to discount the hadeeth but not the quran.

right, exactly

but let's pretend some Sect X decided to do exactly that

how would you think about that sect

It's discouraged to study some hadeeth generally

As there's a lot of false hadeeths

Depending on their reasoning, they might be apostates.

let's say their reasoning was, "our current religious leaders don't explicitly endorse its reading, therefore it is bad to read it, even though we believe the document is genuine"

What does it mean for religious leaders to not endorse its reading? Hadeeth is essential. What they would do is set out the real ones from the false ones, the real ones would be followed as gospel.

For example, the quran would tell you to pray.

well, exactly as I said. Let us say this Sect X of muslims had religious leaders that strongly discourage the reading of the hadeeth, yet believe the whole document to be authentic, and therefore the religious adherents feel like it's some kind of sin or apostasy to even open the hadeeth

But the hadeeth would tell you how you'd go about doing that.

Without hadeeth, and tradition, you wouldn't know how to pray.

these people don't know how at all

in fact almost none of the adherents at this point even know the hadeeth exists

That's an unprecedented situation.

There isn't a hadeeth document. There are compilations.

let's say that occurred to the letter. what would you think?

Various compilations exist

What would I think of who? The leaders?

They would be apostates.

The followers, depending on how much they know and their intentions.

so the leaders would be apostates

Either their leaders would shoulder the blame and they excused, or they would be apostates as well.

The leaders definitely would be.

what would distinguish the one from the other

One being genuinely misled.

but let us say they know of the existence of the hadeeth also, and also believe that it is a genuine document - in fact, they publicly affirm that very fact twice a year in a big meeting.

But don't follow it?

yes