She was not. And The Archers is the lead in her Wiki page.
Ms O’Hanrahan is a rather special case—there aren’t that many actors of that calibre who take radio as seriously as she appears to (she was a Carleton Hobbs laureate). Apparently she was a “lifelong fan” of TA, so presumably would be very familiar with Emma’s voice.
Besides, Emma was recast because Felicity Jones was increasingly unavailable and decided to step down, not just on a whim. There was nothing wrong with Tom Graham’s portrayal (though in an online Q&A session he did rather suggest he wasn’t likely to be a fan of SOC’s editorial style).