Ha, yeah - it gets complicated for sure! PBR stands for Physically Based Rendering, and has more to do with how lighting in a scene reacts with surface properties of a material. How glossy, metallic or rough a material is defines how lighting in the scene will bounce/reflect or otherwise react with that surface.
Texturing comes into play as a means of controlling these properties on a per-pixel basis. So instead of a material having a uniform roughness, for example, a texture can control which areas the material are rougher than others. This can be used to create very detailed and realistic materials, but often in a low-poly style like those shown in the guide for this competition, realism isn't necessarily the goal.
That doesn't mean you can't take advantage of PBR properties though - you could, for instance, have a stone wall material that has a high uniform roughness value so it appears very matte/dull, and a glass material that has low roughness, making it appear shinier/smoother and even reflective when used with environment lighting.
If you're interested in going deeper into learning about PBR, Allegorithmic (creators of Substance Designer and Substance Painter) have some interesting guides here: https://www.allegorithmic.com/pbr-guide