My guess is that it’s a relative of red algae and plants/Viridiplantae, but not quite either.

At least one source mentions it produces lignin or something similar; lignin is present in both clades I mentioned. However since it doesn’t do photosynthesis we can rule out belonging to those clades, I genuinely don’t think evolution would favour ditching phycoerythrin or chlorophyll, so odds are it never developed either.