"Climate change is relatively unimportant" - Only to existential risks. The likelihood of humanity becoming extinct from climate change is low and it will happen slowly enough that we could probably react to it.

"Not able to predict what Coke will do in the future" - True, we can't predict what humanity will be doing in the future, but we can say that either humanity will exist or it won't. If it does exist then that is what we are concerned with. If it doesn't exist then the bad part of existential risk has already been realized.

"Discounting future value of human lives" - You can play with numbers in your DCF formula and find highly unintuitive things like it would be better to kill a billion people a billion years in the future than a million people a million years in the future. That doesn't really seem right. It's also true that the future value of humans isn't "ours" exactly in the way that the future earnings of a company might be, we aren't trying to maximize "our" value, but rather value generally - so I'm not sure discounting formulas like this are relevant.

I find the logic flawed as it seems to imply that extrem suffering of some is justified if it benefits many others. "You can kill a person to harvest its organs to save many" seems implicit, but I just read the summary. The book may contain much more nuance.