It's not just that, this construction:

> This isn't X, this is Y

is a huge ChatGPT signal.

But surely it is only a ChatGPT signal because it was a strong signal in the training data. You need more than one strong signal with that sort of potential for false positives to make a reasonably accurate identification.

If it is, it's one I've neither heard others mention before nor seen often enough myself to consider it a tell (but for the latter, I do use ChatGPT's customisation options).

I love how completely prosaic phrases are now "ChatGPT signals".