Looking at the scss for that preset, I'd agree with Mary that it is for an older version of Boost/Moodle
Older presets tend to have the lines
// Import everything.
while newer ones have the lines
// Import FontAwesome.
// Import All of Bootstrap
// Import Core moodle CSS
The newer ones also often have a theme colors array (though this is not essential, but does make some of the writing easier later on as SASS allows you to loop through them)
Sometimes it can be as simple to update as replacing the imports - BUT THAT IS NOT GUARANTEED! It really does depend what is in the preset, but its certainly a place to start. There were many changes during the development of Bootstrap4 and converting presets is entirely dependent on how the preset was written in the first place.