Apple would always try to repair your computer first. I accidentally dropped and poured a can of coke onto my MacBook and they replaced the whole top case, and screen. The only one not replaced was the bottom panel.
On AC+ for Macs, It's not like iOS devices where they would easily swap out devices.
The probable scenario is that Apple has a limited number of field replacement units but not on site at their bricks-and-mortar stores. There are too many color and different spec combinations to even keep one of each on site at each store location.
They are probably on a shelf somewhere in a regional service center warehouse.
Remember that user-caused damage coverage is not provided under the standard warranty but with the additional AppleCare+ policy. If you purchased your unit in 2017, that AppleCare+ coverage is set to expire this year.
The inventory of ready-to-ship field replacement units is probably very low due to battery freshness concerns. It's not like Apple would want to replace a MacBook with a replacement unit that has a 50% capacity battery from sitting on a shelf unused for two years.