I’m going to be the person who does this I guess. Algae is an alternative term outside of the standard domain/kingdom taxonomy, so it actually spans multiple kingdoms. Most green, red, and brown algae are in the plantae kingdom, i.e. plants. Other algae can be protists or bacteria. The shared trait is that they are non-land photosynthetic organisms. Fossil fuels can be divided into coal (mostly formed from land plants), oil (algae/plankton), and natural gas (algae/plankton). Still classification I guess, but not what people would think when reading this.

While we often gravitate towards putting things precisely into categories, it's important to remember that classifications systems are structures imposed on continuous and complex phenomena for pragmatic purposes. They should not be conflated with the phenomena itself.

It's good to correct misconceptions, but sometimes over focusing on categories distracts from the meaning or sentiment being conveyed, in which case it may be better to be generous with partially correct statements and let it go.

but algae are plants

Lol. Thanks. Yeah, I was grouping that in mentally but I guess I didn't use the right term.