Wrong. RGB is the way to go for t-shirts, from any place that is professional. They use a program, or a filter in Photoshop called ColorCrunch. This color seperates an RGB image, into individual spot colors to closely simulate CMYK printing on a t-shirt.
Check with the printer FIRST. For my money, CMYK shirt printing is horrible looking. I would ALWAYS request the image to be ColorCrunched.
You weren't entirely wrong. I have seen people use a standard CMYK seperation for shirt screening, and iv'e never been to impressed. There's different ways to do this, so it just depends on the printer, but normally, you are very correct.