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The arguments in that thread are true.
Someone thought that they had to convert the file to MULTICHANNEL mode, which is totally unnecessary. That's why you're getting the error message from the PSD file. It's simply saying, in a roundabout way, that you're using an unsupported color mode.
You don't describe which channels are in the file, but it's probably either a grayscale channel + spot channel or CMYK + spot channel. So you just create a grayscale channel and add a spot channel, or start with a CMYK file and add a spot channel. No problem with these in the PSD.
Yeah OK, this is a multichannel PSD, so basically it was created wrong?
It was probably created by someone who learned 10-15 years ago what worked then. They learned in the days of QuarkXPress and DCS files. (Same world I came from...long, long ago.) And they're still doing it.
And, it's not that the PSD file doesn't support Multichannel mode. It's that, for whatever reason, InDesign doesn't support PSD with Multichannel mode.
So, what works in InDesign is just add a spot channel to a grayscale file, or add a spot channel to a CMYK file, and leave it as PSD. That works in InDesign.