I think there must be some swatches in the document that are not set to RGB.
InDesign documents aren’t “in” a specific colour mode, any given colour can be RGB, CMYK, LAB etc.
But interactive PDFs in Acrobat are always RGB only. They can’t be anything else, and InDesign converts all colours to RGB when exporting to interactive PDF.
I was getting regular comments from users about the fact that the colours in their interactive PDFs did not look the same as in InDesign. The reason was always that they had used non-RGB colours in InDesign which were getting converted to RGB on export.
So I added this warning to make users aware that a conversion will be taking place.