CLIP doesn't do captioning, it just generates embeddings. And it's contrastive, so it would work poorly for this kind of task: anything 'relational' falls apart immediately. (See for example the DALL-E 2 results for these kinds of captions/tasks.)
It's almost certainly a VQ-VAE-style encoding of the image itself into a sequence of tokens, as was done by DALL-E 1, CM3, Gato and a whole bunch of more recent models. It's the very obvious thing to do, and their context window is more than large enough now.
This way the model would also be able to generate images, I would also be curious how they handle images with different aspect ratios (and maybe resolution so it can read well on papers).
It's almost certainly a VQ-VAE-style encoding of the image itself into a sequence of tokens, as was done by DALL-E 1, CM3, Gato and a whole bunch of more recent models. It's the very obvious thing to do, and their context window is more than large enough now.