Yes, ERZ is the only zone in SRB2 with 2D segments, but I don't understand why it would be necessary to introduce 2D gameplay earlier on in the game, or how it would be beneficial. It would be like introducing reverse gravity in CEZ, or lava in GFZ. It wouldn't make sense, and nothing would be gained from it. There's no point in introducing a gameplay element before it's actually going to be used.
What is necessary is to introduce a new gameplay concept in a comparatively safe and simple environment before using it in more difficult scenario, but I see no reason why that can't be done in the same level. Of the three 2D segments in ERZ, none start out particularly difficult or complex, leaving the player with enough time to figure out what's going on.