Worry not, as those is not really 3D data embedded in the GlTF.
Having access to your models (as a staff member), I checked the chair of your second screenshot in https://gltf-viewer.donmccurdy.com/ (if you did not know about it, it is a very useful viewer to check for GlTF files validity), and saw no difference from Sketchfab at first:
But opening it in Blender revealed what you were experiencing: the “skeleton” option in PlayCanvas apparently does not display the “skeleton” as is often meant in 3D - an ensemble of “bones” linked together made for animating models - but rather the hierarchy of your scene.
See the following screenshot from Blender which reveals the meshes and their hierarchy (I’ve highlighted in yellow the lines showing the relations between empty nodes and your meshes, which are the ones shown in PlayCanvas) :
The fact that the pivot-point is not centered in the exported GlTF is not linked to those lines, but rather to the objects you have used an opacity material on. By default, most viewers set the pivot-point at the center of all the meshes of the scene, even the ones which you don’t see as they are transparent (but the viewers don’t consider this).
This behaviour is the same on Sketchfab, and you’ll notice it if you double-click somewhere in the viewer or press the space bar while looking at the model: the view will seem to not be centered, because it is actually centered on all the meshes of the scene (easy to see by turning the wireframe on for instance):
By the way, if you did not know about bones and skeleton, I’d definitely advise you to turn on the “Bones” filter in the model inspector for this model for instance to have an idea of what this means:
Hope this will give you some clues !