I am pretty sure none of the oneplus phones have "user" replaceable batteries. The Guardian also states the Oneplus 8 Pro has a replaceable battery but I have that phone and it really doesn't.
Like all devices the manufacturer can replace the battery for a price. Batteries whether genuine or not are not expensive items, the labour etc is which is why I expect the labour price isn't included in One Plus's replacement costs
The rear cover is stuck on with an adhesive and the new 9 pro the battery is split into 2 parts. What I think they mean is oneplus can replace the battery. Not that the battery is replaceable by the user :/