Decay in landfills is anaerobic and produces methane, which has a higher impact on warming than CO2.
Also, composting has a bunch of other no-brainer benefits. Compost reduces demand for fertilizer, can substantially reduce the waste steam, and improves soil quality.
It's not clear to me where you're getting that from. The page neither mentions the word anaerobic nor say that composting doesn't also release the exact same gasses just somewhere else. At best it leans on the reader feeling an emotional difference between the words "decompose" and "decay".
So, yes, they’re roughly equivalent. And things decay faster as compost, so you’d actually be increasing climate change by composting all organic waste.
Methane is mostly a problem when it can’t be captured and burned (ex: cow farts, a serious problem in preventing climate change).
Note that I still support composting. I just don’t think it can be argued from a climate change perspective.