Would you prefer they complicate the discussion by trying to explain what they are? My understanding has always been that they have many plant-like characteristics, but not all. In fact, they are even a part of the plant kingdom.
https://www.sciencedaily.com/releases/2007/06/070619182508.h...
Some are, some aren't. Upon further research it does appear the type used in this algal flour are related to plants but many algae species aren't closely related to plants at all.
https://en.wikipedia.org/wiki/Viridiplantae