How should we interpret Notion Thief's “first card drawn in the draw step”? Is it only the one drawn due to the draw step's turn action, or should it be read literally?

For a complete example, if dredge my first draw while I control an untapped Howling Mine, would my opponent's Notion Thief steal my Mine draw?

Notion Thief's ability should be interpreted literally: if the card being drawn would be the first card that player would draw during their draw step, the Thief's ability does not apply. It doesn't matter why that card's being drawn.

If you dredge your normal draw with a Howling Mine on the battlefield, your opponent's Notion Thief will not steal your Howling Mine draw, because it's the first card you're drawing during your draw step.