Definitely check with the employer, but if they are specifically requesting CNA, you probably need to have it.
Things I learned in CNA class that I didn't in EMT... hm... perineal care, catheter care, bedpans, assisting with ADLs, measuring I&O, making beds, etc.
You may use some EMT skills in the ED but you'll also likely be responsible for CNA stuff like making beds.
Yeah, it may just be wiping butts and taking care of catheters but considering how devastating decubitus ulcers can be and how common catheter-associated UTIs are... it's important that it's done right.