But how do you know that this address doesn't belong to some external service?
Yes, as a rule, services assign an address to each user and don't always use the same one.
If the address belongs to an external service or another party making P2P transactions, we will see a pattern of random transactions, not regular transactions with just a few accounts.
I know the transaction evidence isn't strong enough to prove a connection between several accounts, but from the transactions found, it could indicate the possibility of other evidence being found. If not, some accounts wouldn't bother moving their coins to another address first before sending them to an exchange address.
If you trace the addresses of the captured accounts, it seems like what impulse709 said isn’t nonsense.
I still have more than 200 accounts available.