Exactly.
This was my first thought when I read the question. The address always existed.
And anyone can prove it just by searching in any block explorer an address created offline. Your address was already there, because it is valid.
You cannot just enter anything because there is a checksum (details
here). So if you enter an address 1111111111111111111111111 the checksum is invalid and that address simply cannot exist.
Otherwise it would require some kind of "ownership lock", for example: if you get a private key which creates a given address, you may mark it as "taken". But that idea does not have much sense, for example in the case of HD wallets you would have to mark thousands of addresses as 'yours'.
In other words - you do not know if someone has already an access to the address as long as there is no OUT transaction (or IN, as usually no-one requests or makes transaction to uncorrelated address).