The point being made is that if batteries can be replaced without specialized tools and training, the chances of that being done could be higher, potentially leading to longer usage time and reduced e-waste.
Consider that modern Li-ion batteries are better than older Li-ion batteries (and much better than nickel-metal-hydrides). The need for user-replaceable batteries in modern phones is on par with (or realistically a lot lower) than the need for user-replaceable screens.
My point is that things are rarely obvious. As you say, it "could". It is not obvious that it will make a difference and it might also increase the materials needed on both phones and battery.
I think the EU and European countries have much bigger fish to fry, including with regards to the environment.