I would focus on getting the threat rasters created, where pixel values of 1 are considered a threat and 0 are considered not a threat. The easiest approach to creating these from the LULC might be to use the QGIS raster calculator tool.
As for buffering, perhaps the term buffer has been misleading. It might make more sense to think of it as stopping your threats short of the LULC edge by the greatest Max Distance of your threats. Ultimately, if your threats run up against the edge of the LULC and beyond (in the real world) you are not getting a fair representation of how those habitat are actually affected.
I have annotated the attached image to help demonstrate this. Here we have a LULC map (a cropping from the habitat quality sample data) with two rasters overlaid: 1) Roads as threats in black and 2) Urban as threats in yellow. Note that the road threats stop short of the edge of the LULC by 3km (the Max Distance of Road Threats), this is what we mean by buffering. In this instance your Area of Interest is the to the edge of the LULC. We can confidently say that given the threat layers all our LULC and habitats have been properly affected by the threats.
Similarly, as seen in the 2nd attached image, if we let our threats run to the edge of our LULC we need to understand that our Area of Interest will be the Max Distance of our edge threats from the edge of the LULC, represented by the purple box. Inside this box, we know that LULC habitat have all been properly affected by the given threats.
Sorry for the confusion and our struggle to make this as clear as possible. I’m going to add @jdouglass again, just make sure I am getting this correctly