That’s not true. They may replace the phone sometimes but I’ve only ever had them replace the battery.

I've had 4 battery replacements and Apple has never done anything other than exchange the whole phone.

I've had 2 battery replacements and both times they only replaced the battery.

That has not been my experience. They will swap the battery as long as there is no reason to believe it has damaged the phone (if it has swelled at all, they replace the phone).