Decided to test this myself, I used memreport -full as described here and it appears that as expected, you do indeed save a lot of texture memory by using force volumetric.
Default (using Surface Lightmaps for static meshes)
Total TEXTUREGROUP_Lightmap size: InMem= 85.38 MB OnDisk= 85.38 MB
Force Volumetric
Total TEXTUREGROUP_Lightmap size: InMem= 0.38 MB OnDisk= 0.38 MB
Default
0.219MB - Texture memory 3D - STAT_TextureMemory3D - STATGROUP_RHI - STATCAT_Advanced
138.449MB - Texture memory 2D - STAT_TextureMemory2D - STATGROUP_RHI - STATCAT_Advanced
1.062MB - Render target memory Cube - STAT_RenderTargetMemoryCube - STATGROUP_RHI - STATCAT_Advanced
1.438MB - Render target memory 3D - STAT_RenderTargetMemory3D - STATGROUP_RHI - STATCAT_Advanced
424.250MB - Render target memory 2D - STAT_RenderTargetMemory2D - STATGROUP_RHI - STATCAT_Advanced
605.305MB total
Force Volumetric
0.219MB - Texture memory 3D - STAT_TextureMemory3D - STATGROUP_RHI - STATCAT_Advanced
53.004MB - Texture memory 2D - STAT_TextureMemory2D - STATGROUP_RHI - STATCAT_Advanced
1.062MB - Render target memory Cube - STAT_RenderTargetMemoryCube - STATGROUP_RHI - STATCAT_Advanced
1.438MB - Render target memory 3D - STAT_RenderTargetMemory3D - STATGROUP_RHI - STATCAT_Advanced
424.250MB - Render target memory 2D - STAT_RenderTargetMemory2D - STATGROUP_RHI - STATCAT_Advanced
520.431MB total