That's a good catch, but H2's spectral lines are still discrete. It's not that gasses can't absorb photons, it's that they can only do so at very specific frequencies.

To simplify the argument, you could use Helium instead. Here is a (discrete!) list of its spectral lines: https://physics.nist.gov/PhysRefData/Handbook/Tables/heliumt...

And here is a chart of its (continuous!) index of refraction: https://refractiveindex.info/?shelf=main&book=He&page=Mansfi...