It's hard to say whether or not the device has been liquid damaged from that photo alone. The other side of the logic board may be liquid damaged, or the internal mechanisms in the top case may be liquid damaged, or another component which isn't fully visible in the picture may be damaged. Liquid damage can often occur without the user knowing—the device may have been left in a humid area, there may have been a spill that wasn't noticed or seen by the user, the device may have been set in liquid accidentally, etc. If you're skeptical of their diagnosis, you should ask them to provide proof of the liquid damage or explain in further detail which components are damaged.
Note also that the famous keyboard issue you are referring to is called the "butterfly keyboard" which Apple recognized as a design defect and had a repair program for, but your 16-inch 2019 MBP does not have a butterfly keyboard and does not suffer from that same design defect. The malfunctioning keyboard on your model is most likely due to some kind of internal damage, probably liquid damage that you weren't aware of.
Best of luck!