Earlier quoted context omitted.
"Bouncing" can happen in cryptocurrency world as well, it's called "sending to an invalid address". It just happens to be that the address-space is so big you don't really know what address has a real physical person behind it or not, or yet even. Try sending cryptocurrency to an invalid address and you'll see that the wallet will reject sending it, just like email bouncing.
Most people setting up mailservers don’t consider a catch-all forwarding to /dev/null a valid inbox. And no sane mailserver software forwards to /dev/null by default if you don’t explicitly tell it what to do when it receives email it isn’t supposed to receive. A “valid” address locking up funds sent to it without recourse is /dev/null.
The user in the submission did not send funds to a invalid address. The address is valid, as otherwise funds wouldn't be able to be sent to it (the wallet would not allow you, nor the protocol, nor the miner/validators). The address happens to belong to a contract, that can also hold funds, similarly to accounts.
Now, every address/account/contract has a private-key behind it, that allows the owner of the private-key to transfer out of the address/account/contract, but it's impossible to know if the owner actually still has the private-key.
Similarly to how you can't know if john@example.com actually has access to his email account (maybe he forgot his password?), you can't know if an address actually has the possibility of moving the funds out of the address, as the private-key can have been thrown/forgotten/lost.