The law requires "reasonable accommodations" to provide a equivalent experience. Expecting perfection would not be reasonable. Also with heavy accents, the hearing audience probably doesn't understand every word so if some words in the transcript were wrong, the hearing impaired have the same experience.

There's nothing special about video, the law can also require podcasts and other audio-only content to provide transcripts. But the law doesn't apply to everything and everybody, it applies to "places of public accommodation," and many podcasts are personal projects, to which it doesn't apply. The legal precedent for what is a place of public accommodation is evolving, even including anything online is not universally held, I don't know what counts; having an ad spot in a podcast is probably not sufficient to suddenly make it a business to which the law applies. It almost certainly applies to an operation like Gimlet.