Surely this refers to decomposition and not living plants. Your link doesn't appear to finger plants as the culprit at all.
Only a plant cells containing chlorophyll and exposed to sunlight will photosynthesise. Notably at night there isn’t much sunlight, so normal respiration dominates a plants gas conversion processes, and results in them producing net CO2.
Based on the graphs in the blog post, I assume GP is placing the CO2 production on plants because it the CO2 levels peak at night, then return to normal every morning.