I, too, find that VERY hard to believe!
Tell that to the folks in Florida & on Lake Victoria battling water hyacinth, or the folks in the Southeast watching kudzu overcome everything in it's path.
You really don’t have to get to that level of detail in calculating biomass. We know how much energy the earth receives in sunlight, we know what percentage is converted to biomass by plants, and we know the quantity used.
What’s left over includes your kudzu and water weeds. I’m not including ocean biomass, which is not available for energy conversion. Maybe in some future time.
Plants convert about one tenth of one percent of solar energy into usable biomass. Even the worst solsr cells are a hundred times more efficient. I have seen arguments that genetic engineering could produce more productive plants.
I’m just saying that with current technology, biomass could not replace more than ten percent of fossil fuel.