“Alarmist” is relative. Some folks might be okay with mass extinction across our planet over the very brief period of a century or two, so long as some humans survive to carry the species on into the future.
Others are looking at the mass extinction events happening right now, like the die-off of our coral reefs around the planet (see “Chasing Coral” on Netflix!) and saying to themselves that what we see happening today is more than enough cause for alarm.
Yes, over-population is a problem. Also, capitalism and cheap energy that exacerbates climate change is a problem. They’re related, but each can be addressed separately.