The sun is _slowly_ brightening - this is happening on timescales of tens to hundreds of millions of years. The fact that CO2 was significantly higher tens or hundreds of millions of years ago is not super-relevant to today's conditions, but it helped keep temperatures bearable in the distant past, when the sun was fainter. We are adding CO2 at a rate that is essentially instantaneous compared to the effects of this solar evolution, they are even still extremely quick on the much shorter (tens of thousands of years) timescales of the Milankovitch-cycles (which are the orbital cycles which are the underlying cause of our glacial-interglacial variation in the past few million years)
What interests us at this time is what we are doing to our atmosphere over a period of tens to hundreds of years, and what effect that has on timescales of tens, hundreds and thousands of years - even if we humans stop all of our CO2 emissions (except breathing of course), the increased concentration versus "before" will be considerable thousands of years into the future, as will the effects of that increase on climate.
So stating "still at the extreme lower end of historic levels" is technically correct, but practically misleading, as it suggests there's nothing wrong with CO2-levels.