How to restore from .wt files?
Some sort of benevolent magic occurred after doing the following:

Remove MongoDB Community Edition as a Windows Service, see here.  

sc.exe delete MongoDB


Then when I went into my favourite MongoDB GUI (Robo3T), the databases were just there!  

So i ran:

mongodump  


To get a "proper" dump of all my databases.  

I'm confused how this could happen because the databases are still at their original location:

C:\data\

And the new MongoDB installation data location was:

C:\Program Files\MongoDB\Server\4.0\data\    

¯_(ツ)_/¯

Atleast I have a proper mongodump backup now.