Yes, I agree that the Bitcoin Testnet was the first altcoin, theoretically and chronologically. However, when it comes to use cases, I disagree, as the Bitcoin Testnet was essentially created for experimentation, as it's name.
Regardless, whether Bitcoin Testnet is the first altcoin or not isn't an important discussion, because people will have their own definition & image of the first altcoin, just like what I exemplified in the first paragraph.