“ Portable batteries incorporated in appliances shall be readily removable and replaceable by the end-user or by independent operators during the lifetime of the appliance, if the batteries have a shorter lifetime than the appliance, or at the latest at the end of the lifetime of the appliance. A battery is readily replaceable where, after its removal from an appliance, it can be substituted by a similar battery, without affecting the functioning or the performance of that appliance.”

https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A52020PC0798&qid=1703805580803

So we see here that batteries must be replaceable without affecting the function of the device. Yet waterproofing is important. What seems more likely to me is that batteries need to be replaceable without opening the entire device and therefore destroying liquid protections as per the proposed law. Easiest way to do that would be something similar to a SIM card tray where a hidden button is pressed to release the battery to swap it. The designers would have to go out of their way to make this process difficult, which the EU also doesn’t want, to avoid making them swappable. And that feature is attractive. Knowing Apple though, it’ll be harder on the base models or batteries will cost too much.