Those distinctions aren't clear from the linked article. Do you have a source that Netflix doesn't have to caption their video productions but NBC does?

Teacher's Prep sounds like an online-only course program. Why couldn't they pull this alleged online-only Netflix loophole?

The requirement the GP was referring to comes from the combination of the Telecommunications Act of 1996 and the 21st Century Communications and Video Accessibility Act (CVAA) rather than the ADA. The Telecommunications Act requires "video programming distributors" to provide closed captioning for most televised video programming (televised being the operative word here). The CVAA extended the requirement so that it also applies to televised video programming distributed on the internet. But according to the FCC's consumer guide on the CVAA, it "does not cover programs shown only on the Internet"[1].

The CVAA doesn't apply to Teachers Test Prep any more than it does to Netflix. But Title III of the ADA includes a section on "Examinations and courses" that states "Any private entity that offers examinations or courses related to applications, licensing, certification, or credentialing for secondary or postsecondary education, professional, or trade purposes shall offer such examinations or courses in a place and manner accessible to persons with disabilities or offer alternative accessible arrangements for such individuals"[2]. That's what the Justice Department found them to be in violation of. The fundamental problem was that their courses were inaccessible; lack of captioning (or other "alternative accessible arrangements") was what caused them to be in violation but wasn't a violation in and of itself.

[1] https://www.fcc.gov/consumers/guides/21st-century-communicat...

[2] https://www.ecfr.gov/current/title-28/chapter-I/part-36/subp...

Thank you for writing this up! Very good summary of the relevant laws and regs.