I have been looking at this data regularly.
First, these #Bitcoins are not that readily avaible, they are owned by customers.
To what extent an exchange wants or needs to backup their customer coins I don't know, but they can't be all sold.
The fact this number is declining is never the less an important one.
The core signal is that less people store their coins on an exchange and thus take self custody of their coin.
One of the real supply sources is coins available on OTC.
This data is not fully transparent.
All, or most, large buys take place OTC.
If that supply runs dry we will see a more clear/honest price discovery since buyers would need to go on exchanges to buy.
For my that is the largest and most liquid supply, those willing to sell on exchanges when the price goes up.
For now the balance between large buys OTC and exchange flows is not that easy to fully grasp.
For sure some fiddling is going on ?