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That wasn't my statement. I was quoting somebody who was quoting somebody. I'm afraid the fonts I used were confusing.

However, I will answer your question. Mars' atmosphere is almost entirely CO2, like Venus'. The big difference is that Venus has about 90 times the total atmospheric density as Earth, while Mars is about 1/170th.

While Mars has very little atmosphere, it still has about 12 times the density of CO2 as Earth. Yet Mars is still much colder than Earth. All three planets are warmer than they would be without atmospheres, Mars is slightly warmer, Earth quite a bit warmer, and Venus "hellishly" warmer.

The big difference is that amount of atmosphere on each planet rather than the particular gasses that make it up.