Prescriptivism vs. descriptivism. Taxonomically, you're right — kelp is a macroalgae, not a plant. But most people don't really care (and don't need to care) about this taxonomical distinction in their day-to-day life. It has a stem and leaves, it eats via photosynthesis, it's rooted to a single spot, and it's green-ish — as far as I'm concerned, that's a plant!

I generally find that defining words descriptively (based on how most people use them most of the time) rather than prescriptively (based on how institutions or niche groups use them) is a good way to avoid conflict, both on the internet and IRL!