I disagree with the author at a surface level; we can retain much more than 90% of what we read. The curious can look up deep reading strategies, e.g., those summarized by Benjamin Keep.

At a deeper level, though, there’s truth that we have limited time here; we can’t read everything.

90% of what you intend to keep, or 90% of everything you read for pleasure? The latter is much more to remember.