It's certainly true. By definition. If the bar for general intelligence is being smarter than the median human, 50% of people won't reach the threshold for general intelligence. (And if the bar is beating the median in every cognitive test, then a much smaller fraction of people would qualify.)
People don't have a consistent definition of AGI, and the definitions have changed over the past couple years, but I think most people have settled on it meaning at least as smart as humans in every cognitive area. But that has to be compared to dumb people, not median. We don't want to say that regular people don't have general intelligence.