You’re either worshipping God, or worshipping something else. I think that’s the point. There is no neutrality.
Those who refuse to worship the Christian God suppress their knowledge of Him specifically (not just some generic philosophical deity). This means Deists, Unitarians, Atheists, Polytheists, etc. all qualify under this blanket statement as suppressing the knowledge of the truth, for this reason: what can be known about God is plainly revealed in the creation. But rather than submit to Him, they go after other gods, whether they be idols, themselves, the creation, etc. I think Paul is driving a wedge between those who worship the Christian God and those who don’t.
Mind you, even those who now worship God were once suppressors of the truth, so there is no reason to think that Christians are any better than those outside the fold; however, this does give us perspective in reaching out to others, because we realize they know the truth, but they don’t want to admit it because God has given them over to follow after gods of their own imaginings.