The law does not have a severability clause. If any parts of it goes, it is gone.
That is true, but just as with the Hobby Lobby case, the challenge is not to the statute, but to something the executive has done with or to it. In the case of Hobby Lobby, SecHHS Sibelius directed that abortifacient drugs be considered "basic health care" -- there is no statutory requirement in the "ACA" for them to be covered, the conflict with religious liberty was created entirely by the use of the "as the Secretary shall direct" powers which Congress delegated to the SecHSS. In this case, the challenge is to powers the IRS has arrogated to itself, arguably in violation of both the plain meaning of the statute and its legislative history. If the administration loses on this, the law is still in place, it's just that what's left is an even less workable train-wreck of a law than the thing "fixed" by the IRS's administrative hubis currently is.