SpaceX uses slight variants of their 1st stage rocket engines in vacuum, so they don't really need this kind of testing. And this test rig is 10x larger than SpaceX's current vacuum Merlin 1D needs.
This testing gets you valuable information about performance and stress characteristics which can allow you to make improvements to the rocket, either by changing its shape for better efficiency or by making it lighter where it turns out to be over engineered. SpaceX hasn't been doing this testing yet because it hasn't been necessary and building a test stand too expensive. Now they could.