Part of the modern USB PD spec is that the two sides of the negotiation usually have a set of voltage/amperage pairings that they'll do (the highest aggregate pair(s) giving you the wattage of the adapter). Sometimes the things won't find a configuration that matches and so it won't charge. That's why, for eg., you can't charge the Nintendo Switch Dock with a macbook charger but can charge the switch itself. The dock is really picky (basically only does the pair the official nintendo adapter wants) and so is the macbook charger. The switch itself is more flexible though.

That said I would have thought the toothbrush would just do normal old USB charging, and that should work on a macbook charger no matter what I'm pretty sure? But maybe not.

Edit to add note: I think the most commonly missing voltage is 12v, so your toothbrush might want that.