Does it add billions of years? That's not saying that life could have arisen and survived since 15 million years after the Big Bang.
The paper implies that it only adds millions of years, not billions.
a new regime of habitability made possible for a few Myr by the uniform CMB radiation
Once the CMB cools down enough with the expansion of the Universe, the Goldilock conditions disappear. The CMB temperature is roughly inversely proportional to the age of the Universe, so 300K at 15 million years becomes just 150K 15 million years later.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.