I’m sure many do come from customers. But the idea that exchanges never buy bitcoin from each other to maintain their stocks seems quite a fantastical claim to me.
If they did see arbitrage opportunities they are engaging in risk (there is timing risk for example) and I'm not even sure they could legally do this as deposits should be ringfenced.
This fact exposes them to theft and something akin to a bank run, where withdrawals can exceed deposits, something that can never happen on the NYSE because it doesn’t hold inventory.