You can achieve open-vocabulary classification by making the final weights in the softmax come from a category encoder instead of being fixed learned weights. So instead of

softmax(encode(input)*learned_weights)

You have

softmax(encode(input)*encode(categories))

I'm not sure if Jev does it this way, but it's how you get open-vocabulary zero-shot image classification with models like CLIP [1].

[1] https://openai.com/index/clip/