Mm, don't you just need to retrain the embedding layer for the new tokenizer? I agree it seems likely this is like a stopgap new model release or a distillation of mythos or something while they get a better mythos release in place. But there are some things that look really different than mythos in the model card, e.g. the number of tokens it uses at different effort levels.

Maybe it's an abandoned candidate "5.0" model that mythos beat out.