“In general all morphological forms can be found in low PO4 concentrations ….” That along with neutral to acid ph and lots of light.
I think the real weakness with this theory is that it doesn't explain why you get a worse algae outbreak in bright light.
For algae, as a photosynthesising organism, light equates to energy for growth.
However, I've quoted an experiment previously which found that in terms of growth, red algae grew most in lower light intensity (65 PAR vs 300 PAR).
PO4 is essential to photosynthesising plants and algae. It is used in the Calvin cycle to produce the compound ATP, which organisms use to store energy.
When heterotrophic bacteria break down organics, such as plant material, they release PO4 into the water column, which is re-absorbed by other organisms.
PO4 can frequently be a limiting nutrient. An increase in organics, may mean an increase in heterotrophic bacteria and hence an increase in released phosphates in the water column.
Removal of a PO4 limitation due to a heterotrophic bacteria outbreak may lead to algae being able to store more energy through photosynthesis, which it in turn means it may be able to release organic carbon for the benefit of the heterotrophic bacteria.
However, as your quote says, red algae can be found in PO4 limited environments.
Low pH may affect the autotrophic bacteria, leading to an increase in ammonia concentration.