It is incorrect. They are not plants. I am curious why you think it is correct to call protists plants? It is not just taxonomy, but ecology, life history, tissue structures, and so much more. Some plants do photosynthesis. Some algae do photosynthesis. Not all things that do photosynthesis are plants. Not all things that do not do photosynthesis are not plants.
It is incorrect. They are not plants. I am curious why you think it is correct to call protists plants? It is not just taxonomy, but ecology, life history, tissue structures, and so much more. Some plants do photosynthesis. Some algae do photosynthesis. Not all things that do photosynthesis are plants. Not all things that do not do photosynthesis are not plants.