- "tokens" or "assets" are around in Bitcoins since ...
If you change the content of the OP_RETURN in
this tranasction to send 2000 USDT instead of 1000 USDT, the transaction is still a perfectly valid bitcoin transaction, will be relayed and mined. However it will no longer be a valid Tether/Omni transaction, and the Tether amount will not be transferred to the secondary address anymore.
Since none of these protocols are actually supported or enforced by the bitcoin protocol, we can't say "these 'tokens', etc. exist in bitcoin"
You're partly right, at least we can say "tokens are not part of the Bitcoin protocol". What's part of the Bitcoin protocol however is OP_RETURN, which allows storing of arbitrary data which can be ignored by the Bitcoin validation algorithm.
And token protocols are enforced in a similar way than Bitcoin transactions: They use Bitcoin PoW's decentralized timestamping quality to determine the transaction order. So while you can create a valid Bitcoin transaction which is not a valid token transaction, once you are using the token's extension protocol, the token's rules become enforced by the Bitcoin protocol plus the extension software.
I think that's more or less a discussion about semantics, though
