This is what the service manual that Microsoft published says:[br]
[image|2896344]
And the "Illustrated Service Parts List" section make no mention of a bottom case assembly/backplate cover, where the battery would be found if it can't be separated. If a component is not listed as replaceable, then you won't be able to order it from a device reseller:
https://learn.microsoft.com/en-us/surface/surface-customer-self-repair-surface