Water is a tricky thing!
Pure H2O is not the bad guy! Its whats in it that is!
So if you spilled distilled water leaving the system off until it fully dried the amount of damage will be very little as only the stuff it picks up within your system would be present.
- If on the other hand you spilled a cup of ocean water, sea water, rain water, bottled spring water or even tap water from the faucet, all have stuff in them that can harm your system - Mostly Salts & Minerals.
- Lets say you spilled a cup of orange juice or a can of soda besides the stuff above you have Acids & Sugars.
Remember all of this stuff won't evaporate with the water! So it will stay on your system and short out or corrode parts over time.
So what is the bottomline here? You likely need to still open your system and clean the areas the spill took place on. I'm suspecting it was mostly into the keyboard and trackpad both of these are the more difficult to get to and clean. Mostly we just replace them as its just easier. Which is why they recommended you replace the uppercase assembly.
For now how about just making sure the logic board is OK. You'll need to follow this IFIXIT guide to take it out: MacBook Pro 13" Retina Display Early 2015 Logic Board Replacement. Once out use cotton swabs (Q-Tips) to mop up any visible spills and use distilled water to sip down the areas. I would then go over the same areas again with 85% isopropyl alcohol to help speed up the drying process and to remove any oils that may still ben present.
Let the parts fully dry and hopefully the keyboard and trackpad work for now. You'll likely need to replace the uppercase at some point. If you see anything that you question take some pictures and post them here for us to see. Its also possible there are some other parts that need replacing.