Yes, that is exactly the intended workflow.
The Unity Extension uses AO Lightmapper as the baking/export pipeline from inside Unity. It can process Unity meshes or prefabs, generate the AO/baked texture data, create the required UV2 when needed, and output a standard GLB/glTF asset that you can then use outside Unity.
So yes: you can use Unity as the preparation tool, generate the baked AO version, export the resulting GLB, and load it later in Three.js using GLTFLoader.
Just to clarify: this is not exporting Unity’s native scene lightmaps in the traditional Unity lightmapping sense. AO Lightmapper generates asset-based baked Ambient Occlusion/lightmap textures for the model itself, which makes the asset portable and suitable for Three.js, Babylon.js, React Three Fiber, etc.
The generated GLB is saved back into your Unity project folder, so you can also copy/use that file directly in your web project.