While there is a known issue with the Display Cable for these 2018 models, but it usually affects the backlight although it can sometimes affect the video signal as well. Even the LCD Panel itself can have this issue once in a while which has nothing to do with this specific model...just a general failure that can sometimes occur. It is hard to say what caused the fault.
Regarding possible signs of liquid damage...some techs just are not that great at seeing signs of liquid damage. It does take some experience to detect some liquid damage as it may only show up as a very tiny spot of corrosion...which may be no larger than the size of a dot. It takes me time to train techs to see detect liquid damage since it can be a quite small spots of it. Some techs can also misidentify some things as liquid damage, when it may be something else entirely (happened to our organization once when we mailed a laptop to Apple for repair).
If your laptop does have signs of liquid inside the laptop, I don't think it has anything to do with the current display issue. However, if Apple detects signs of liquid inside the laptop, then Apple will consider the laptop has accidental liquid damage.
Hope this helps to explain some of the discrepancy and why even you may not necessarily notice some very small signs of corrosion. There are times when even I can miss some small signs of corrosion even though I have years of experiencing looking for it. Some corrosion may only be visible with a magnifier and even then it may still be hard to see.
The $350 price seems like it may be an unauthorized repair shop since that is about the cost of the part from Apple when an AASP performs a repair (of course an AASP must add labor & possibly sales tax as well). The problem with this is that with the 2018+ Macs, Apple uses a special service provider only utility to program the Logic Board/T2 security chip to work with some parts. For some laptops this includes the Display Assembly, but only for some laptops. The programming may also program some color & HDR calibration information as well according to Apple. Without the part serial numbers being properly associated with the laptop, there is a possibility it could affect other system functionality such as being able to stream HD videos due to HDCP requirements many streaming services may require. I don't recall if this particular laptop requires a "programming" the Logic Board with the Display's part serial number.
An official Apple Authorized Service Provider who actually orders a replacement part from Apple and installs the part themselves in their own shop may provide a slightly lower price for the Display replacement even if there are signs of liquid. I would expect a repair performed this way by an AASP would be about $500 US. Keep in mind Apple Stores and many AASPS will just mail a laptop out to an Apple mail-in repair depot where repairs are based on Tiers with set prices...when accidental damage is detected the cost of those Tiers is increased substantially. The same does not happen when an AASP replaces just a single part such as a Display if the part is ordered & installed at their shop.
