Depends what you mean by ‘accurate’. The over/under exposure warning will accurately show what pixels are close to white/black after processing (i.e. at the end of the pixelpipe). You could theoretically take a pure white (‘overexposed’) image and reduce exposure to bring the entire image down to mid-grey. The indicator would tell you that nothing was overexposed at all.
If you’re looking for an indicator as to whether your original raw image is over/under exposed, you should use the raw over/under exposure warning.
In the case of your channel mixer example above, the channel mixer module clipped information out of your image but because subsequent processing steps took that clipped information and made it non-white, it didn’t show as overexposed.