It says every grain of sand contains a thousand worlds though. Even in other translations that's exactly what it says. I don't know why that wouldn't include the grains of sand within the subworlds they just mentioned.
If they had said every grain of sand in our world contains another thousand worlds then that point would stick.
It isn't exactly what it says. It is not that every grain of sand contains the world (the sentence just says "the Buddha's world in a grain of sand" with no indication of anything like recursion, subworlds, or anything like that).
I think you just take it a bit too litteraly and outstretch it way beyond the meaning of the sentence.
I think that
the translation below, which is a bit more understandable, shows it quite well (not saying the one you're are using is bad tho, since I'll use its stuff right after).
When you know that there are no things and no mind
Then you are a Buddha with a true mind and a Dharma body.
A Dharma−bodied Buddha has no form;
A single divine light contains the ten thousand images.
The bodiless body is the true body.
The imageless image is the real image.
It is not material, not empty, and not non−empty;
It does not come or go, nor does it return.
It is not different nor the same, it neither is nor isn't.
It can't be thrown away or caught, nor seen or heard.
The inner and outer divine light are everywhere the same;
A Buddha−kingdom can be found in a grain of sand.
A grain of sand can hold a thousand worlds (chilioscom);
In a single body and mind,
all dharmas are the same.
For wisdom, the secret of no−mind is essential,
To be unsullied and unobstructed is to be pure of karma.
When you do no good and do no evil,
You become a Kasyapa Buddha.
Clearly the subject of this whole thing isn't about anything like "it's sand all the way down", but rather something like "to see the world in a grain of sand".
Also I found the translation you were using, and this part is indicated to be a borrowed poem, rather than a cosmological explanation from
"Chapter 14, p. 153: The prefatorial poem beginning with the line, “The Buddha is Mind and the Mind is Buddha .” As indicated in part I of this introduction, this piece directly reworks an ode by Zhang Boduan (982/4?–1082), reputed founder of the southern lineage of the Quanzhen Order. See the Wuzhen pian , gathered in the collection, Xiuzhen shishu in DZ 263, 4: 746."
And given how this very same chapter, starts with an allegory "the six robbers disappear" (referring to the six sense of the body), I would be veeeery careful about how you litteral you would want to take things.
So, unless this borrowed poem is something meant to convey the message that "there's an infinite recursion of worlds each within its own grain of sand" (which also wouldn't make much sense, since I highly doubt regular humans are meant to be super powerful 1-B beings), I highly doubt it's giving a tier any time soon.